I do not think that a watermark would be a good idea, though I understand the desire for an expanded free plan.

The example I have in mind is the Unity game engine, where free users have a splash screen in their exported games. Since most users who aren't paying are also not making great games, Unity's name and reputation is now attached to solely those bad games.

Similarly with the generated images, adding a watermark to (some of the) free ones will mean that the images you have your name on are the ones that are being generated by users who aren't paying; either because they're not making enough to justify it, or they're making bad content that you don't really want to be the main thing with your name on it out there.