I put mine on the front of the photo, but the one's I use are very small they're 'page markers' so not full sized post it's.

So far none of them have left any marks, but the longer the wait you have the more chance of damage I guess, it's something I may have to change if any photos in the future do get damaged by what I do

Recently I've been putting a post it note on the photo just saying 'Please personalize 'to Lorri' if you have the time'.... So far it's worked ;)

Good luck!