If someone wants to post someone's photo in his profile he can open another profile photo save photo on the disk and then upload as their own, and it will look the same as on your screenshot.
I think that if someone break rules on the site then his profile must banned and deleted.
Favourite photos is some alternative way to implement profile photos, but not the best way. We'll create separate module or functionality for profile photos, or maybe it will be some 3rd-party module for this, which will work better then just favourite photos.
Yes, and your example demonstrates the problem.
Somewhere you explain that since in any case it is possible to do it illegally, then as much as the site proposes it in a legal way (Legal? Must still see ...)
To a photo thief, I can explain to him that what he did is not legal, but in this case, he will answer me, so why does your site offer me to do it?
I repeat your system is great for all photos, but poorly suited for personal and private photos.
The problem is that there is no way to distinguish between the two.
As I said, I took an extreme example, but I'm afraid that nobody will appreciate finding his picture on someone's profile especially on the profile of someone he does not like, whatever the reason.
Admin can not remove people's profiles simply because they do not appreciate and only for this problem.
Look at a simple request for friendship, why do you accept it or why do you refuse it? Because there are reasons or not.
I can refuse a request for friendship, still happy, but not that I publish my head on a profile? Or is the difference, for a third-party visitors, he will think those two are friends because they have their photo side by side.
I think that's a bigger problem than you think. When something does not like a site, 5% of people will talk to the admin, 95% will delete their account.
I may have an idea, I do not know if it is good and achievable, but I submit it anyway below.
Thus the author could decide for each object whether he accepts or not whether he can be favored by him alone or by everyone.
So admin can tell author, uncheck the photos you do not want to see elsewhere and even do it himself in some cases.