images shouldn't be an issue since the game will automatically ignore them if they're not formatted properly (i.e. too big / small); it could be an issue of having too many characters loaded at once since the game has a limit of how many can be in the game at once or else it will freeze
currently the game only supports up to 2 characters being loaded at once due to memory constraints, though I'd say test it out with a single character for now to see if there's a specific character you have that's causing the issue
