Ok, assuming that those are the exact images you're using, unless the image is huge on the page, I was correct--it's because you're resizing them by setting an explicit height and width smaller than the actual image. The PNG is 137x74, so unless you're using width="137" height="74", you're resizing them, causing the distortion you're seeing.
IE7 handles PNG alpha channels well enough (IE6 won't without the previously mentioned hacks), but it does an awful job of downscaling images on the fly, which is what you're seeing.
Make a PNG or GIF that is the actual pixel dimensions you want your final image to be on the page, and use that--it'll work.
(Also, even if downscaling worked properly, you've still got weird sizes--that PNG is 137x74 pixels, and since 137 is a prime, you're going to be changing the aspect ratio no matter what values you use.)