Currently Being ModeratedFeb 11, 2013 8:18 PM (in response to tallpole)
Ok... I figured it out on my own...
Messages will regularly (~1 hour I think) change all the avatar/profile images it can for all the various jabber, AIM, etc (including gchat, since it's jabber protocol). I thought it was changing it to my Address book or "contacts" profile, or my login images, but these are independent of the one Messages was using (atleast for me). Here's how I fixed this for me:
Go to Mac OS Menu Bar (when focussed in Messages):
Messages -> Change My Picture...
Then drag an image you want to use into one of the "recents" tab, use the "other" tab, or take a picture of yourself using the web cam. Once you've selected a photo, you can crop it within the image window, or leave it alone. When you're ready, just click "done" button.
That's seeming to fix this PITA for me... why Apple decided my Mac's account image, Contacts profile, and Messages avatar image should all be different... beats me.