[Cardinal Rule]

Well the first thing I see is what you already mentioned - seems very two dimensional. Drop-shadows or a contrast-colored outline might add some depth?

Of these I like the one on post 5 the best, but the difficulty with using that color yellow for the horse is that it becomes harder to see where it is up against the white background. If the horse had a more three dimensional look it might stand out more. The theme of that design is very good though.