Well, it took me 5 days to figure out and still I haven't - how to mix up misture and background image (added with nodes) correctly? The problem I'm talking about one can see even on this image, but for some more results you can check this thread I've created on the blender.stackexhange: http://blender.stackexchange.com/questions/8223/adding-background-to-the-landscape-and-mixing-with-mist.
Problem means one can see clouds through the island. I understand that, according to mist creating principles, objects in the distance become more alpha-transparent, but there should be possibility to avoid that. You can find more information about my research using link I gave
