SEE!!! There Ed goes again being an EXTREMELY FRIENDLY PERSON!!! AND showing off why he's a MOD, and I'm not!
By the way, Ed. You're correct! For the Mac platform, press and hold control then click the desired userbar, then a menu should should show up with an option that says "Copy Image Address", or something to that effect. You click that option then, using the img bookend brackets shown in Ed's code box, paste the image url into the signature portion of your profile.