My avatar is PNG. Do I have to do something special to make the white parts disappear?
Did you just download it that way or did you edit it yourself? If you edited it, you'd have to save the white parts as transparent. Whether or not your browser renders the transparent parts properly might be a different issue (I seem to remember older versions of IE not supporting this).
As for editing it, Paint won't do transparency, but GIMP and Photoshop should.
Edit:I tested it out and made a transparent version. Would you like me to email it to you?