I have the same problem but i just found out, at least for me, that it is not WordPress related. In my case, i uploaded some PNG files to my server and for some reason, the images just won’t display in Firefox 2.0.0.5 or Internet Explorer 6.0.29 on Windows XP, but it will display fine on Safari 3.0.2!!! I finally figured out that it was because of how those PNG files were saved. What i did to fix this problem was to open the image up in Microsoft Photo Editor (comes in XP) and then save it as another PNG file.
