Canon Characters should be banned, even if it's the OP's fanart. "My Version of" Canon Characters shouldn't be allowed as it's just fanfic territory. Canon characters but species swapped shouldn't be allowed either; it's still fanfic territory. Playable characters should not be allowed (IE the above: Frisk, Hollow Knight, Pokemon Trainers that have canon appearances: ex Red, Blue, Kris, etc).
Customizable characters should be allowed (IE MMO, Stardew, KHux, Pokemon trainers from X/Y and recent games where you can change the looks,) basically if they have no canon name and allow customization in-game so they don't look like every other MC that's played. Any characters who are nothing but multiple screencaps shouldn't be allowed in general. This should fall in similar line to Dreamselfies/etc with proper credit too.
Canon Species: case-by-case. No official art or fanart that wasn't created by/for you. Must have some changes in order to be valid, be it palette tweaks, accessories, unique markings. If canon images work for it, it probably doesn't need a gallery.
Canon characters or Canon Species should be allowed as non-characters, however, if mixed with original characters. If my FFXV comrades OC learned how to cook from Ignis and I have images of them together in the kitchen, or my original Pokemon Trainer with canon Pokemon designs eloped with Brock who also has canon Pokemon and I drew them on a whimsical romance to the Pokaribbean, that'd be fine in their gallery.
Canon settings are fine as long as they're not using screencaps from shows/games/etc or other official images/copyrights.
Sales of canon-derived characters should be banned, too. This includes Pokemon, Steven Universe, and the like. Selling IP that isn't your IP to begin with is kinda skeevy imho.
I saw faceclaims mentioned and I will say Absolutely Ban because those are images created by someone else. Movie/TV screencaps should be banned, any photography done by someone is copyright to either the photographer or the people the photographer hired. If you don't have explicit permission from everyone involved, don't use it. Also this with moodboards, since RedRarebit reminded me.
Excessively late edit:
Bad Faceclaims = "here's a bunch of images of Tom Cruise from his website, he is the face of my OC. I made a bunch of icons from photos of him for rp-ing on tumblr, here they are, all credited to me."
Passable Faceclaims = "Here's this link to an image of Tom Cruise, his facial features align with my idea of this OC and I'm using it temporarily until I can get a proper reference." or "If I had to pick an actor for my OC, it would be Tom Cruise, here's a link to Tom Cruise's website for some images."
Bad Moodboards, shouldn't be allowed = "Here's this moodboard I found on Pinterest." "Here's this moodboard made of images I cropped from Pinterest, I'm credited as the maker." "I made this moodboard, all images are copyright Google/Pinterest/Tumblr" "Here's this moodboard: Credits here: [line of short links, sources are professional or hobbiest photographers who do not have blatant permissions given for editing/redistribution]
Good Moodboards, should be allowed = "Here's this moodboard I (or someone here) made. Credits here: [line of short links going outward to sources, all of which are free stock images or otherwise CC type images]"
Again, I know in the rules it says "don't upload images you don't have permission to use" but faceclaims and moodboards are very common, especially the latter, and break the rules. Canon Characters and the like are similar. Just because an image is posted to the internet does not mean it's free for the taking. Just because it's rehosted somewhere, doesn't mean you can rehost it here. Pinterest is not a source. We Heart It is not a source. Google is not a source.