Mail has had the ability to show pics for anyone all along, provided those pics were in Address Book (easy to add). In 10.2, something broke--Address Book started storing the pics in a different location, but Mail wasn't updated to look there. There are a couple of programs that will copy them to the new location for you.
See:
http://www.versiontracker.com/dyn/moreinfo/macosx/16740
http://www.dasgenie.com/Pages/Software/macosx.php
from:
http://forums.macosxhints.com/showthread.php?s=&threadid=6415

I'm not sure about this, but I wouldn't be surprised if you had your own pic in your entry in Address Book, and added your vcard to your .sig, then other Mail users might see your pic when they viewed messages from you. Shouldn't be too hard to test.