That’s a tricky one, I don’t think it’s ‘cheating’ but it’s probably not a great idea. You have to think, how would they feel if they DID find out in these situations, and are you prepared to take that risk?
Having said that, if you make it properly anonymous, and there’s no risk of them finding out, maybe it’s okay? This includes not capturing tatts/backgrounds etc or details in a story that are obviously yours! Also if you submit a picture, never, ever submit it elsewhere where it could be identified, google image search is a scary monster.
So yep, sorry, definitely your call on this one, let your conscience decide.