IE for Windows doesn't support 24-bit transparent PNGs normally, but with some hacking you can use a roundabout method to display them on IE5.5+:

http://support.microsoft.com/default.aspx?scid=kb;en-us;294714

I managed to get the logo in MonoBook displaying with transparency by sticking this hack into the IEFixes.css:

#portlet-logo a, #portlet-logo a:hover { background-image: none; cursor: hand; filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src='/ ~brion/rel1.3/stylesheets/monobook/wiki.png',sizingMethod='crop'); }

Unfortunately it seems to want a URL relative to the HTML page, not the stylesheet, so we can't hard-code it in the file. It could perhapsbe hacked in at runtime by JavaScript (yuck!), or by a <style> block embedded in every page with a conditional comment (yuck!)

For <img>s in the page, there's a very nice JavaScript fragment which will walk through the page and replace them with hacked up <span>s at load time without much other effort. May be worth looking at: http://homepage.ntlworld.com/bobosola/pngtestfixed.htm

-- brion vibber (brion @ pobox.com)