I am not sure, but read somewhere on HN Google related article comments that what Google does is first load all image search page with own encoded compressed thumbnails, enlarged by css, and then async fetches the real images, replaces the thumbnails, and scales down by css to show as same size of thumnail.

Edit: in this exact thread, another user too made same observation at https://news.ycombinator.com/item?id=16390022