Overall, I would like to start by saying this is an excellent effort, with a good, clear objective. However, there are points that could be improved.
To make it more realistic, I feel that you could add some droplets of rain onto the image surface, which would give the illusion that the screen is wet. Also, So lighting in the background would also add to the effect of the image, helping it to stand out.
The horse, on the other hand, is pretty much perfect, However, I suggest toning the spots brightness down a little more, which would help to make the image seem a little more realistic.
As well as that, I think that the clouds at the very top of the image could be given some more detail, as right now it seems to be quite basic in and around that area. However, As I already said, overall I really like this piece, and hope to see more like this from you soon.