i wanted to do the exact same thing using apocalypse's portraits (which are too large by default, you need to resize then but they still look good enough).
i never spent time on it because i just deemed it impossible (and it was also lower-ish priority, compared to some other thing is still have to finish).
very cool idea.
ideally people should be enabled to add portraits packs***: anime characters, famous people (actors, politicians, singers), your friends-relatives (your grandma killing sectoids), avatars from the community in which you're making a let's play, or whatever the modders come up with.
*** but to check if the feature makes sense, some testing portraits should be made, to see if they still look good when resized that small. apocalypse's portraits are ok because they're black and white; random photos or images (and colored too) might look like shit.