[onion2k]

Image size could include things for retina displays, higher standard resolutions, CSS sprites and so on. Again though, a lot of images in sites are standard reusable assets that are held in the browser cache between visits - it's just editorial and user-contributed content that won't get much benefit from a well configured http server.

Sidenote: I found out about kraken.io on HN a while ago. I use it for all my images now. Usually gets at least 5% reduction on a well created picture, and 50%+ on a bad one. (No affiliate, just like it)

[jol]

I don't think that CSS sprites should be bigger, given that css is improving, I don't know about the sample analysed, but usually the editorial and user-contributed content images (as well as ads) take much more bandwidth than style related images, unless the site has exotic design. aside: kranken.io is nice