That's a lovely piece!!! I absolutely adore it. I was going to say that you could add a shadow of the horse onto the jump, but honestly, I wouldn't do that. Too much chance of ruining a piece that is just adorable as is.
the ONLY suggestion I would have is to possibley reposition your signiture so that it's a bit higher and to the left. And maybe keep it a tiny bit more horizontal. Your signiture becomes part of the overall composition, so having it in that slightly blank area to the left helps balance the picture, AND, by moving it up a bit, will give room for framing and not have it be cut off.
I would get that one professionally scanned so that you can print a bunch of them.=A print of it would look really cool modge podged (it's a kind of clear glue you can paint over the top to do decopage) onto a tack box or something horsey.