Just wanted to call attention to the news post I added earlier today with new concept art from Ryan Church and Josh Nizzi on their respective websites. Nizzi's site in particular has some interesting robot designs that didn't get used in the movie like "trailer armor" for Optimus, and head designs for the 12 original Prime's rather than just 7.
Also, although I didn't include it in that news post, if you'd like see concept art from the first movie, Ben Procter has some on his website.