OK - I think this photo most predominantly shows three things: First, that Ian McKellan is no horseman, but also that he has really skinny legs. But on the positive side, it shows that he does have the guts to try to do his own riding!
It also shows his shoes...
Another thing I noticed. The film seemed to imply that the lord of all horses was ridden bare back. It has no reigns. And when it ran up to Gandalf it had no saddle........... Wassup with the stirrups?