From the Painting Guide: This is a lovely dual portrait, particularly of the white horse (I'm guessing Buck) where the deceptively simple washes against the areas left white have created such a magnificent, three-dimensional feeling. Composition wise, the two horses are beautifully placed -- they dominate the painting as you'd expect, but balance each other while at the same time having 'space' on the right-hand side to look into. I'm presuming that Rusty's face has been cut off at the bottom when you squared up the photo, because otherwise it makes for a strange composition. Also that there's more of the tree trunk on the left than just that sliver, for the same reason.

One thing I do find incongruous however, is the shadow of the tree in the foreground which is slightly lighter in tone than the surrounding ground/leaves; I'd expect it to be darker.