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Is the image a PNG file?

IE does not currently support PNG Alpha Transparency. The new IE 7 will, I think.

There are workarounds.
-You can save the PNG as a 256 color PNG.
-You can use an IE DXImageTransform filter to load the thing instead. This is a bit of a PITA, really. Google around for more info.

Or you can do what most people do and ignore it. Tell people that IE sucks and to switch to Firefox.