It is easy to fault someone elses image, its not so easy to come up with the sort of image that you have produced.
I think that the idea is really good and the effort that you have put into it is teriffic.
Now for the silly bits:
The rails on both sloping sides of the bridge have been sliced off showing that the bridge is a cut and paste job. Mabe you could remove them?
The top of the right hand side of the bridge is partly missing.
The way that the dog is positioned looks as if its laying on a slope,
The slanting line of the grey tiles draws the eye away from the image, it could be cropped out.
The image is very good, please do not let my comments put you off. I believe that making an image on a computer is just the same as making one in a studio using props. The image has to be seen in the mind first. Something that many people can not do.
I hope that I have helped.