New pages, same old header problem

I recreated my Benzene web page (which was the freshman year Chemistry project that got me started on the web development path). I think I've figured out a way for the pages to link to images and not run into problems when I switch themes; for now, the pretty benzene molecule images are visible, as are the for-a-long-time-broken images in my Links page. The Links images weren't that important, though; the benzene images are much more helpful.

Meanwhile, in header-land: my PNG header image doesn't work on IE 6 on XP, but it does on IE 6 on Windows 2000. And I'm told it works but is very small in IE 6 on a different XP machine. I don't know what is going on, but I may just lose the transparency aspects that I like so much and use a JPEG. My test JPEG definitely works on IE and Firefox, I just hate to remove something that works in Firefox (and works well) so that IE and Firefox can be a little less elegant. Compromises....