Ummm ... what was wrong with just using text? Does it really need to be in a image?
PNG in Windows IE is the website which explains the PNG problem and has the workaround script, with good instruction on how to use it. It makes PNGs with alpha channel work properly in Internet Explorer versions 5 and 6. It will let you use the image that you were first using, and have it work properly in most browsers. I'd use this method. I can help you put it into the website.
If you want to use different images, then you should give special images to Internet Explorer versions 5 and 6 only, because they have the problem. Give Firefox and other browser normal images (with transparency). I can make a script for this if you want.
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