Start by reading this:
OOTP Baseball Manual - Team and League Logos
I think all the stuff from ootp 11 carries over to ootp 12 regarding logos (someone correct me if I'm wrong).
Then you can get logos to start with here:
OOTP Logos
and here:
http://www.ootpdevelopments.com/boar...mes-logos.html
After a while you could even try to make your own logos with GIMP or Photoshop. Good luck and enjoy.