I got 9/15 in the end. Some of them were easy to guess just because the picture quality was different. But a lot of them could be guessed by how different the elements in the picture were from each other. The AI art tended to look unique, but one part of the picture looked exactly like the other. AI, as it is today, draws from a set of existing examples to derive a style and create a new picture entirely based on that style. It isn't adding any ideas of it's own.