Not too bad! They cutting out is very neatly done.
In the future, you may want to line up light sources. Such as the sorrel with the cloudy background - he's real shiny, but the photo in the background is rather dark. It would make those look more realistic.
Thanks for the tips! That makes a whole lot of sense. Oh and I use aviary.com it's a free website you sign up for and you can make really neat stuff. I personally use the phoenix application on the site.