Opera (and others I think) does this with JPEG quality. They mirror the image files as far lower quality JPEG so as to improve speed for those on limited bandwidth. You can of course request the full files.
Correct. If you enable Turbo in Opera Mobile or desktop, all traffic is sent through a proxy server which converts images into WebP format. The quality is better than when we used jpegs and the file sizes are much smaller.
(Disclaimer: I work for Opera)