@Ashley218
I host a lot of exchange students who stay at my private home listing for a whole semester. To some extent, if they are staying for more than a month I think I could understand wanting to personalize the space a bit. Also, sometimes when I travel, I see a mirror hung somewhere that totally does not make sense to me. If it's a couple days I'd let it alone, but if it's for a couple months, I'd probably think about moving the mirror somewhere more convenient for me.
Still.......I have to admit hanging your own picture is a bit odd. (People travel with their own pictures for the walls???)
As long as the guest hasn't done any damage, I think I'd let it go and maybe casually as if they'd like the decoration stored elsewhere and if there's anything else in the room they'd like removed. As long as any of the changes they make are temporary and easy to switch it back to the way you had it...... as a host, I think I'd try to be cool about it 🙂