Well, that's how you build a training set. But you still have to identify features and use an algorithm that basically goes "users identify images with feature

Well, that's how you build a training set. But you still have to identify features and use an algorithm that basically goes "users identify images with features A, B, and C as ads, and they identify images with features X, Y, and Z as not ads, so put positive weights on A, B, and C and negative weights on X, Y, and Z, and block anything with a negative enough score".