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This is a lovely picture, my dear. I have been wanting to critiqueit for a while.
First of all, the horse is extreamly beautiful. Capturing muscle on an all white horse is hard to pull off, but you did great. The main and tail also looks nice. I really love the face, it looks almost real. It doesn't really look like it's running, which takes a pinch away from the over all effect.
I like your idea of the stars, but personally it didn't work for me. Points for creativity though.
I think the clouds and the tree are well done, and I love the area were the tree is coming out of the clouds.
Background and planet is wonderful, I could stare at them all day. Good job.