No they don't, they have four legs in every picture. Look at Nefermandias, he is a four-legged brown. The dragon on page 169 has his/her hind legs curled up underneath the front legs.
Ahhh, I see it now. Completely missed Nefermandias.
By the way everyone there is also a preview of the Player's Handbook Heroes Series 2 off the same page. Not quite complete yet, though.
DDM Spoilers by Greyhaze