Akamai supports it as a transparent speed optimization for clients who want it. My employers website is fairly image-heavy and we use Akamai’s Image Manager to optimize images for us. The first time an image is fetched by their CDN they analyze it to optimize it for size, compression, and image type, and all the rendered versions are cached on their CDN. When a client requests the image Akamai will look at the characteristics of the device and serve the best optimized version of the image.