Ok, first off IE can't handle transparent PNG files. You can change them over to gif files, and then IE can handle them. Or you can call in the PNG files through your CSS files, as below - just define it in a class like you're allready calling.
The !important tage will only renger in complient browsers - not IE, so it ignoers it and goes on to the seconed line.
I think what's happening here is that IE comes along to thses transparent png files, doesn't know how to handle them, and just dies. Instead of trying to render the rest of hte page, or give an error, it just dies. that is so helpful...
Anyways - does this fix your problem?