Use PNG unless you have a specific reason to choose another format. Select JPEG when response latency matters and you do not need transparency; OpenAI’s Image Generation guide says JPEG is faster than PNG. Select WebP when your receiving system supports it and efficient web delivery matters. For transparent backgrounds in the Responses API image-generation tool, choose PNG or WebP because JPEG is unsupported.
PNG, JPEG or WebP: the short decision
| Requirement | Choose | Why and limits |
|---|---|---|
| Default or broad compatibility | PNG | PNG is the documented Image API default. Your destination application still has to support the file. |
| Lower latency and no transparency | JPEG | OpenAI documents JPEG as faster than PNG. No numerical speed difference is specified. |
| Transparent background | PNG or WebP | The Responses API image-generation tool does not support JPEG with transparent output. |
| Website or other network delivery | WebP, if supported | WebP supports lossy and lossless compression and alpha transparency and is designed for fast transfer. It is not guaranteed to be smaller in every image or accepted by every workflow. |
| Adjustable compression | JPEG or WebP | output_compression accepts a documented range of 0–100 for these GPT Image outputs. It does not apply to PNG. |
File format is separate from how the API returns the data. With GPT Image, output_format selects png, jpeg or webp. For DALL·E 2 and DALL·E 3, response_format selects a hosted url or b64_json response envelope; it is not a file-format switch.
What each format means for an image-generation workflow
PNG: the safe starting point
PNG is the default output format in the Image API. It is a sensible first choice for illustrations with sharp edges, text, interface mockups, logos and images that may be edited again. It also supports transparency. The default does not mean universal compatibility: check the application that will consume the returned bytes before shipping an automated pipeline.
JPEG: choose speed over transparency
OpenAI’s guide states: “Using jpeg is faster than png, so you should prioritize this format if latency is a concern.” That is a qualitative recommendation, not a promised percentage or service-level target. JPEG is appropriate for opaque photographs and similar images when a quicker response is more valuable than lossless output or an alpha channel. Do not request it for a transparent background in the Responses API tool.
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WebP: a delivery-oriented option
WebP can use lossy or lossless compression and can carry alpha transparency. RFC 9649 describes it as optimized for transfer over networks such as websites. Those capabilities make it useful for web publishing, but they do not prove that every generated image will be smaller than PNG or JPEG. Confirm that your browser, CMS, image pipeline, mobile app or downstream API accepts WebP.
Keep file format separate from response encoding
Two parameters are easy to confuse because both sound like they describe an output. They solve different problems:
output_format: the bytes’ image format for GPT Image:png,jpegorwebp.response_format: a DALL·E 2 or DALL·E 3 choice betweenurlandb64_json. The documented URLs remain valid for 60 minutes. This parameter is not supported for GPT Image, which returns base64-encoded images.
Consequently, changing response_format will not turn a PNG into WebP, and adding output_format to a DALL·E request is not a substitute for checking that model family’s reference. Confirm the endpoint and model documentation before copying parameters between API surfaces.
Requesting a format in code
The essential change is the format field in the GPT Image request. Keep the rest of your prompt and authentication code unchanged, then decode or save the returned base64 image according to the SDK or endpoint you use.
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Python parameter object
request = {
"prompt": "A clean editorial illustration of a mountain observatory at dawn",
"output_format": "webp",
"output_compression": 82
}
# Send request to your GPT Image integration.
# Save the returned base64 image bytes with a .webp extension.Use png without output_compression for the default path. For JPEG, set output_format to jpeg and choose a compression value only when your quality and size requirements are understood.
JavaScript or Node.js object
const request = {
prompt: 'A clean editorial illustration of a mountain observatory at dawn',
output_format: 'jpeg',
output_compression: 80
};
// Pass request to your GPT Image client and write the returned bytes as .jpeg.cURL request body
curl -X POST "$IMAGE_ENDPOINT"
-H "Authorization: Bearer $OPENAI_API_KEY"
-H "Content-Type: application/json"
-d '{
"prompt": "A clean editorial illustration of a mountain observatory at dawn",
"output_format": "png"
}'Here IMAGE_ENDPOINT represents the image-generation endpoint already used by your integration. Keeping the endpoint in an environment variable avoids accidentally applying GPT Image parameters to a DALL·E request.
How to choose compression
For GPT Image JPEG and WebP outputs, the documented output_compression range is 0–100. Treat it as a control exposed by the API, not as a guaranteed file-size percentage or a universal quality score. Test representative prompts and inspect the actual files in your destination workflow. PNG does not support this option; sending it with PNG can produce a validation error or be ignored depending on the endpoint version.
- Decide whether transparency is required.
- Verify that the receiving system accepts the candidate format.
- If latency is the priority and the image is opaque, try JPEG.
- If web delivery and alpha transparency matter, try WebP.
- Compare visual quality and actual byte size using the same prompts, dimensions and quality settings.
- Record the chosen format with the asset metadata so later processing does not infer it from a filename alone.
Transparency, backgrounds and downstream handling
Ask for a transparent background only when the destination can preserve alpha. In the Responses API image-generation tool, use PNG or WebP; JPEG cannot represent that transparent output there. A transparent image may display against a checkerboard in an editor but appear against a solid color in a CMS or browser preview. Test the final consumer rather than trusting the preview.
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If you do not need transparency, an opaque background can simplify compatibility and reduce surprises in systems that flatten alpha. Conversely, converting a transparent PNG to JPEG later requires choosing a background color; it cannot recover transparency.
Latency, size and compatibility trade-offs
When latency dominates
Use JPEG as the documented first option when every request must return sooner and transparency is irrelevant. OpenAI does not publish a numerical benchmark in the cited guide, so do not promise a fixed improvement. Network time, prompt complexity, image dimensions and service load can still dominate the total request time.
When delivery cost or bandwidth dominates
WebP may reduce transfer overhead, but RFC 9649’s network-optimization description is not a guarantee for your particular image. Measure the generated files and verify support in your CDN, CMS, app or partner API. Keep PNG available as a fallback when a consumer rejects WebP.
When editing or archival matters
PNG is generally the least surprising interchange choice among the documented options because it is the default and supports transparency. Preserve the original returned bytes before making derivative JPEG or WebP versions.
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Troubleshooting common format errors
“The transparent background is rejected”
Cause: JPEG was requested with transparent output in the Responses API tool. Fix: request PNG or WebP and confirm that the destination preserves alpha.
“Compression is invalid”
Cause: the value is outside 0–100, or compression was supplied for PNG. Fix: use an integer in the documented range with JPEG or WebP only.
“The parameter is unknown”
Cause: a GPT Image parameter was copied to a DALL·E request, or the endpoint’s current version differs. Fix: identify the model family, then follow that endpoint’s current reference. DALL·E’s response_format controls URL versus base64 JSON, not PNG versus JPEG.
“The downloaded file will not open”
Cause: base64 text was saved as if it were binary, or the file extension does not match the returned bytes. Fix: base64-decode the image field, write binary bytes, and use the extension matching output_format.
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Cause: a DALL·E hosted URL exceeded its documented 60-minute validity. Fix: download it promptly or request b64_json when your workflow needs to retain the bytes itself.
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpSee the ScreenshotNeo documentation for the other 63 capture options, including full-page and element shots, device presets, retina scale, custom CSS or JavaScript, waiting rules, blocking, cookies, headers, geolocation, caching, signed links, webhooks and bulk capture. The Free plan includes 1,000 screenshots each month with no card; paid plans start at $5 for 3,000. Create a free ScreenshotNeo account.
Frequently Asked Questions
Can I request both PNG and WebP in one image-generation call?
Choose one output format per request. If you need multiple derivatives, generate once and convert or request additional outputs in separate calls while preserving the original bytes.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesDoes a higher output_compression value always mean a smaller file?
The API documents a 0–100 control for JPEG and WebP, but the material does not define a universal size or quality relationship. Measure the files produced by your own prompts and settings.
Should I use response_format for GPT Image?
No. The reference documents response_format for DALL·E 2 and DALL·E 3. GPT Image uses output_format and returns base64-encoded images.
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