This has to be out there somewere but I'll post anyhow. To put a graphic in your mail signature all you do is drag and drop in the signature that you are creating. Great for logos, etc.
[Editor's note: Hmm, don't think this has been posted here before, and I didn't know it was possible!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021209050628171