I know that some CDNs like Akamai will serve it up in cases where it optimizes delivery of the image. They have an add-on service for their customers called Image and Video Manager. Among other things it can compress images to improve performance.

Imagine a website that displays a JPG file that has thousands of colors and a resolution of 10,000x10,000 pixels. That’s terrible for loading across the internet because the human eye can’t distinguish that many colors and most devices have screens smaller than 10k. The website in question could also be putting that image in a frame that’s only 1k in size.

So what Akamai’s service does when it sees this image is that after serving it up the first time it optimizes it to a number of different sizes and formats. It will make multiple copies in different resolutions & file formats, and reduce the number of colors without impacting the visual appearance of the image. The optimizations target popular devices & browsers, so you could end up with a bunch of different sized jpegs, webps, pngs, etc.

Once all those versions have been created they are automatically added to Akamais CDN cache. The next time a person visits that website Akamai will look at the characteristics of the device being used and serve the best optimized version of the image that the device supports. So if you’re running Chrome on a mobile device you might get a 800x800 webp version of the image rather than the original 10,000x10,000 version, for example.