The lengths I go to for you IE 6 users.. (or, browser image resampling/rendering differences)
As part of my job (and my nit-picky nature around quality), I choose to tackle fun problems like this. It'd be nice not to have to do this, but sometimes you have to get creative.
This image shows blown-up (8x normal size) screenshots of how a 75x75 JPEG thumbnail is rendered at native size, and at browser-resized 48x48 sizes in IE 6, Firefox 3 and Safari 3. When shown at non-native sizes, the browser must resize the image itself - and depending on the implementation, the resized image can look really bad.
IE 6 does the worst job of resampling, presumably the drawing method is not bicubic but something closer to linear; thus, the resized image looks pretty crappy.
An interesting workaround is to use IE's AlphaImageLoader activeX filter (assign a transparent .gif to the in question, and then apply the filter with the real URL.) This does appear to use a better resampling method, as illustrated between the first and second small thumbnails.
Firefox 3 and Safari 3 both display the scaled-down image quite nicely.
Also see Opera 9.52, Chrome and IE 8.