Yes, that is The Doctor riding a horse. It seems the seventh season of "Doctor Who" will take Amy, Rory, and The Doctor to the old west.
An old picture of a Beatles jam session contains a man who looks almost exactly like Matt Smith's Doctor. It makes perfect sense when you think about it. If I had a box that gave me access to all of time and space, chilling with the Beatles would be pretty high up on my list of things to do. I'm surprised it took The Doctor until his eleventh generation to get around to it.
Steven Moffat is pretty excited about the photo too.
"Bloody hell!!!" He tweeted, "Clearly we're going to make that episode!! Wonder what it will be like."