Fair use depends very heavily on the nature of the use. A key part of Google's defense was that not only was it not using the entire books to reproduce the ent

Fair use depends very heavily on the nature of the use. A key part of Google's defense was that not only was it not using the entire books to reproduce the entire book, but also that it was taking measures to prevent people from abusing Google's systems to reproduce an entire book. It's a lot of work to emphasis that the impact on the market (in other words, the fourth factor) is as minimal as practicable--and that's the crux of the analysis. When you're instead scanning someone's stock image database to build a tool to generate stock images... the fourth factor is jumping up and down screaming at you "YOU LOSE" and your best defense is that it's not the training, it's the tool built on the training data that is infringing the copyright.