IE6 and IE7 don't scale images nicely in web pages when the images are scaled with css width/height or attribute width/height. I am not sure which algorithm it uses by default, but it's not good.
Scaled images display aliasing artifacts when scaled in these browsers.
Fortunately there's a way to force IE7 to use the bicubic scaling algorithm (which looks very nice) with a simple css rule:
img { -ms-interpolation-mode: bicubic; }
The results are great, and it can be done site-wide with this single rule.
For IE6 you're out of luck as far as I know.