this is also a shot in the dark because i have no idea if it could work?? but as for tagging characters in separate images, perhaps instead have a "character ping system" that relies on the ID of each character submission? so in order to easily link or ping a character to an individual image upload, either just have the person write in the description, or alongside the area that lists the artist, put a "featuring:" and in there you'd write for example, :id5712: to link the characters?
for the "featuring:" addition, i assume it'd be a little like adding to the descriptions of chars themselves and you can add however many characters are featured in the image. of course this sort of thing would only work for characters already on th, but ye <:3