It greatly annoys me when web pages don't resize images properly. In particular, one thing that there seems to be an epidemic of at the moment is web pages that embed images at one resolution and then use styles like "max-width: 100%" in a column narrower than the image. This makes the images look horrible because most browsers (at least IE7 and Firefox 2) resize the images by just dropping columns, causing diagonal lines and curves to be all wavy. At least Firefox 3 gets this right and resamples the image but it's still a waste of bandwidth.
Along similar lines, here is an interesting article about the interactions between gamma correction and image scaling. I hadn't thought about that before (my own image resizing routines always just assumed a linear brightness scale) but this has definitely opened my eyes.