by themselves they could never recognize beauty

Disagree, the first humans to see something beautiful must have recognized it for themselves.

an artist cannot know what is beautiful or not without FIRST some exposure to beautiful things.

True, but after man grasps the essence of what beauty itself is, he can recognize beauty in a multiplicity of things that have nothing to do with the things he had seen before. An AI will not recognize other beautiful images in addition to those previously selected by a human for it, because it does not understand what beauty is.