Embed a PNG in HTML by placing its bytes in a data: URL:
<img src='data:image/png;base64,ENCODED_PNG_BYTES' alt='Description of the image'>The image/png media type identifies the file, base64 declares the encoding, and the comma separates that metadata from the encoded bytes. You can create this value in a browser with FileReader.readAsDataURL(), generate it during a build, or write it directly into a single-file HTML document.
Use the correct PNG data URL format
A PNG data URL follows the grammar data:[media-type][;base64],data. For a PNG, the complete prefix is exactly data:image/png;base64,. The comma is required; everything after it is the Base64 representation of the PNG’s binary bytes.
<img src='data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAAB...' alt='A one-pixel PNG'>Replace the shortened payload above with the complete encoded file. Base64 is case-sensitive, so do not change its letters, numbers, plus signs, slashes, or trailing equals signs. Keep meaningful alternative text in alt so the image remains understandable to people using assistive technology.
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Convert a PNG selected in the browser
For a file chosen by a visitor, the native browser workflow is FileReader.readAsDataURL(). When reading finishes, reader.result is already a complete data URL, including the data:image/png;base64, prefix.
<!doctype html>
<html lang='en'>
<head>
<meta charset='utf-8'>
<title>PNG Base64 preview</title>
</head>
<body>
<label for='pngFile'>Choose a PNG</label>
<input id='pngFile' type='file' accept='image/png'>
<img id='preview' alt='Selected PNG preview'>
<script>
const input = document.querySelector('#pngFile');
const preview = document.querySelector('#preview');
input.addEventListener('change', () => {
const file = input.files[0];
if (!file) return;
const reader = new FileReader();
reader.addEventListener('load', () => {
preview.src = reader.result;
});
reader.readAsDataURL(file);
});
</script>
</body>
</html>Why this code works
accept='image/png'filters the file picker toward PNG files. It is a usability hint, not a substitute for validating the file.input.files[0]retrieves the first selectedFile. The guard handles cancellation or an empty selection.readAsDataURL(file)reads the binary file asynchronously and prepares a data URL.- The
loadhandler assigns that complete URL toimg.src, so the browser decodes and displays it.
Show the generated value or copy it
If another part of your application needs the string, use reader.result inside the load handler. This example displays it in a text area and provides a copy button:
<textarea id='dataUrl' rows='8' cols='80' readonly></textarea>
<button id='copy' type='button'>Copy data URL</button>
<script>
const output = document.querySelector('#dataUrl');
const copy = document.querySelector('#copy');
reader.addEventListener('load', () => {
output.value = reader.result;
preview.src = reader.result;
});
copy.addEventListener('click', async () => {
await navigator.clipboard.writeText(output.value);
});
</script>In a real page, define reader in the same scope as the event handlers. The important point is that the value is already a complete URL; do not Base64-encode it a second time.
Extract only the raw Base64 payload when an API requires it
Some APIs want only the encoded bytes and not the declaration prefix. Remove everything through the first comma:
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const dataUrl = reader.result;
const comma = dataUrl.indexOf(',');
const rawBase64 = comma === -1 ? '' : dataUrl.slice(comma + 1);For a known PNG, a stricter version also checks the prefix before extracting:
const prefix = 'data:image/png;base64,';
if (!dataUrl.startsWith(prefix)) {
throw new Error('The selected file did not produce a PNG data URL');
}
const rawBase64 = dataUrl.slice(prefix.length);Use the full data URL for an HTML src attribute. Use rawBase64 only when the receiving API explicitly asks for the payload without data:*/*;base64,.
Embed a known Base64 value in static HTML
For a self-contained document, paste the complete data URL directly into the image element:
<figure>
<img src='data:image/png;base64,PASTE_THE_COMPLETE_VALUE_HERE' alt='Company logo' width='240' height='80'>
<figcaption>Company logo</figcaption>
</figure>Keep the payload intact when templating or minifying. Accidental truncation, inserted whitespace, or line-wrapping can make the image undecodable. If a build system stores the value in a variable, concatenate the prefix and payload without adding spaces:
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<script>
const pngBase64 = 'ENCODED_PNG_BYTES';
const image = document.createElement('img');
image.alt = 'Generated chart';
image.src = 'data:image/png;base64,' + pngBase64;
document.body.append(image);
</script>Generate the data URL outside the browser
Build scripts are useful when the PNG is part of a single-file artifact, email template, prototype, or generated report. Read the file as bytes, Base64-encode those bytes, and prepend the PNG prefix.
Python
import base64
from pathlib import Path
png_bytes = Path('image.png').read_bytes()
data_url = 'data:image/png;base64,' + base64.b64encode(png_bytes).decode('ascii')
print(data_url)Node.js
import { readFile } from 'node:fs/promises';
const png = await readFile('image.png');
const dataUrl = `data:image/png;base64,${png.toString('base64')}`;
console.log(dataUrl);Both examples preserve the binary bytes and produce a browser-ready URL. If you are writing an HTML file, escape the value according to the templating system you use, then place it in the quoted src attribute.
Data URL versus a normal PNG URL
| Concern | Base64 data URL | Normal image URL |
|---|---|---|
| Document size | Binary data is copied into the HTML, increasing the document. | The HTML stays smaller because the image is separate. |
| Caching | The image is tied to the containing document rather than fetched as an independent asset. | The image can be cached and reused as its own resource. |
| Updates | Changing the image requires changing the document that contains it. | The image can be replaced independently at its URL. |
| Deployment | One HTML file contains everything needed for the image. | You must deploy and serve the HTML and image resource. |
| Readability | The long encoded payload is difficult to inspect or edit. | The short path is easier to recognize in source. |
| Origin and security behavior | Modern browsers treat data: URLs as unique opaque origins, and top-level navigation to them is blocked in modern browsers. |
The image is fetched from its normal document or asset origin. |
Choose a data URL for a small, self-contained asset when portability matters more than document size. Keep a normal URL when caching, independent updates, source readability, or a smaller HTML response is more important. Exact practical size limits vary by browser and should be checked against the documentation for the browsers you support.
Understand btoa() and atob()
btoa() encodes a binary string and atob() decodes one. They are not general arbitrary-Unicode encoders: characters outside the byte range can cause an error, and Unicode text must first be converted to bytes. A PNG is already binary, so FileReader.readAsDataURL() is the safer browser-native path for a File or Blob.
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Do not run btoa() on the complete result of readAsDataURL(). That result is already Base64 plus its data-URL prefix. Encoding it again produces a different string that cannot be used as the original image.
Troubleshoot a broken PNG
The image icon appears, but the PNG does not display
- Check that the value starts with exactly
data:image/png;base64,. - Confirm the comma is present between the prefix and payload.
- Make sure the payload was not truncated, wrapped, or altered by a template.
- Verify that you assigned the complete
reader.result, not an object or an empty variable.
The browser reports an invalid or ignored data URL
A misspelled base64 marker or malformed media parameters may be ignored without a useful error. Use the canonical lowercase prefix and remove unexpected characters before the comma. If you extracted a payload for an API, remember that the HTML image still needs the prefix added back.
The selected file is not a PNG
The accept attribute influences the picker but does not guarantee the file’s contents. Check the file type and, where correctness matters, validate the PNG signature or process the file through an image library before embedding it.
The image works in a preview but not after saving the page
Inspect the generated HTML and compare the saved src value with the value assigned in the browser. Common causes are a templating escape, a missing comma, a shortened string, or whitespace inserted into the payload. Preserve the entire encoded value.
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The HTML became unexpectedly large
That is inherent to inline encoding: the binary is now part of the document. Use a normal PNG URL or an asset pipeline when the image is large, reused across pages, or updated independently.
Performance, accessibility, and security considerations
- Performance: Inline data makes the containing document larger and prevents the image from being managed as an independent cached asset. Keep inline PNGs small and limited to assets that benefit from being self-contained.
- Accessibility: Provide useful
alttext. If the image is purely decorative, use an emptyaltvalue rather than exposing meaningless encoded content to assistive technology. - Integrity: Treat the Base64 string as binary data. Do not normalize characters, insert spaces, or trim meaningful trailing equals signs.
- Navigation: A data URL is intended for embedding; modern browsers block top-level navigation to
data:URLs for security reasons. - Maintenance: Keep the source PNG and a reproducible conversion step in your project so you can regenerate the embedded value when the artwork changes.
Or skip the browser setup
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import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)Node.js
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
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What does the comma in a PNG data URL do?
It separates the media-type and encoding declaration from the Base64 payload. Omitting it makes the browser parse the value incorrectly.
Should I remove the data-URL prefix before putting the value in an HTML image?
No. An HTML src needs the complete data:image/png;base64, prefix. Remove that prefix only when a separate API explicitly requests raw Base64.
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