It's simple:

Red John (His Red Right Hand) http://img29.imageshack.us/img29/7037/4sh2.png
Brett http://images2.wikia.nocookie.net/__cb20100620225057/thementalist/images/4/4e/Brett.png

Look at the noses and the elongated face. From other small glances at RJ (again in His Red Right Hand, dont have an image) when RJ poisons the lady (I forgot her name at the moment) you catch a one frame second of RJ. He has short black hair. Rules out a few right there. 

The remaining suspects have fat short noses and/or more round faces.