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The Quality feature reduces the quality of images delivered through the CDN. It works in combination with at least one other Image Stack operation — such as Compression (WebP or AVIF), Resize, or Crop — and cannot run independently. The origin retains the original image; only the version delivered to end users is affected. The following example shows the same image at four different quality settings. The visual difference is subtle for detailed or non-contrasted images, and more noticeable for images with large solid areas against contrasting backgrounds.
Example of high image quality

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Enable quality

Quality is included in the paid Image Stack option. To use it, enable Image Stack for the CDN resource following the Enable Image stack guide.

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Use quality

Quality can be configured in two ways: via the Gcore Customer Portal to apply a default level to all images matching the rule, or via query parameters to set quality per individual image URL.

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Use quality in the customer portal

1

Open the Image optimization rule

Navigate to the settings of the required CDN resource. Click the Rules tab, then open the Image optimization rule.
CDN resource settings showing OPTIONS and RULES tabs with Create rule button and Image optimization template in the dropdown
2

Select the quality level

In the Image Stack section, select one of the built-in presets: High (95%), Medium (80%), Low (65%), or Custom value. The default is High (95%).To prevent quality reduction for PNG images, enable Enable lossless for PNG. When enabled, the quality setting applies only to JPG images.
The quality value cannot exceed 100%.
Image Stack section showing Image quality radio buttons with High selected and Enable lossless for PNG toggle
3

Save the rule

Click Save changes at the top right corner of the page.

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Use quality via query parameters

Query parameters set quality for individual images by modifying the URL in the website source. 1. Open the website source code. 2. Find the URLs of the images whose quality needs to change. 3. Add the quality query parameter to each URL alongside at least one other Image Stack parameter:
where:
  • value is a number between 1 and 100
  • tool2=value2 represents query parameters for any other Image Stack function such as Compression, Resize, or Crop
For example, to reduce quality to 76%, convert to AVIF format, and crop a 200×200 px area from the center:
The following example shows the same image processed with different quality and fmt parameter combinations:
  • Link: img.jpg
  • Size: 813 KB
  • Quality: 100%
Original image before processing
4. Save the changes in the website source code.

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Check HTTP headers

After processing, the image URL and file extension remain unchanged. Use the HTTP response headers to verify that quality conversion was applied. The X-Img-Operations header lists all conversions performed. If the value includes convert, the image quality was changed.
Browser DevTools Response Headers panel showing X-Img-Operations header with value convert highlighted
If X-Img-Operations does not contain convert and the CDN returns the original-quality image, check the Img-Skip-Reason header. It explains why the operation was skipped — for example, converted image bigger than origin appears when the configured quality value is higher than the original image quality. In this case, combine Quality with another Image Stack feature.
Browser DevTools Response Headers panel showing Img-Skip-Reason header