Additionally, it seems to me that it would be even more difficult depending upon the privacy settings of the possible infringing user profile. If you, as the copyright holder, have restricted access to the user's profile; how would you even know.
Then why worry about it? Its one thing to protect your work so that you can profit from it and to make a living from it, but its another to try and control every aspect to how it is seen or used.
Anyway, putting aside that point, if you don't know about an infringement, you cant do anything about it, its just logistically impossible. Best you can do is to make it difficult to download or copy your images, and or search for your work to make sure your copyright is not being infringed.