

I often use Midjourney for NPC character images for my personal (private) game amongst five or six friends, but I would avoid trying to build a brand using it, even for free supplements. The brand becomes “AI generated,” which anyone can do.
One reason people buy books is for cool, imaginative art made by actual human beings.
I homebrew everything these days, but back when I ran off-the-shelf adventures, I customized and arranged large parts of them as well. I didn’t do it to catch cheaters, but it would be very good for that.
Anyway, I just told players not to read outside sources and be careful about what you search for. I said if you come across outside information once, I’ll chalk it up to an accident, but if it happens repeatedly, I’ll remove you from the group. I didn’t feel the need to explain or justify that.
I had one case of someone searching a name to figure out the spelling, and they ended up spoiling themselves for a major character reveal. They were a good player and they regretted spoiling it for them themselves, and it never happened again.