I ran into a problem where I wasn't able to get my PNG images to properly render in Internet Explorer 7 or 8. I was using the jquery cycle plugin, jquery.malsup.com/cycle/, and I was trying to display the images on an already transparent background, but little did I know, it was an option in the plugin.
The plugin's option page clearly shows this, but I was unable to see where it was documented to make it easier for people with this problem. The jquery cycle options shows that it has many different features. One of which either applies the background to the PNG images or removes it.
Just apply this option to where you are referencng the plugin:
cleartypeNoBg: true
This should instantly resolve your problem.
Note: This isn't a fix for Internet Explorer 6.