This one is safe, may have to ask permission, but I completely changed this from the original. I don't believe I broke any laws.
This one I am unsure about. The man can be traced back to the original. I may have to ask permission before I ever sell it or do anything with it. Unsure.
This one I believe is border-lined. The trees and the man are both from the original, but I completely changed the image so much it would be hard to figure out the true identity of it.
This one I believe is safe. I have the image appropriated with a couple of mine. I took the trees and made it into wall paper. This one I believe is safe and not breaking any laws.
This one is also safe. I just used the texture from the original and laid it over my image. I don't think this is breaking the law.