Internet Explorer up to version 6, while it supports PNG (portable network graphics) images, it does not handle transparency correctly. Instead of blending into the background, the transparent section appears grey. There is an easy javascript way to fix this and you can find out how here: [http://homepage.ntlworld.com/bobosola/pnghowto.htm].

This has to be coded into your web pages in order for the client to see them correctly. This is supposedly fixed in Internet Explorer 7 and is already featured in Firefox.