If you're using PNG it's because Internet Explorer SUCKS at rendering PNGs. IE can only do binary transparency, it cannot do a full alpha channel.
There are two ways around this: you can convert the image in Gimp to indexed color (yuck, but compatible; not suitable for all types of images), or use terrible scary IE hacks to make it render alpha transparency (looks great, but doesn't always work depending on IE version and security settings).
See: http://support.microsoft.com/default.aspx?scid=kb;en-us;294714
There are some scripts to automate use of this hack such as: http://homepage.ntlworld.com/bobosola/index.htm
-- brion vibber (brion @ pobox.com)
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