Yes this is a good idea, and not currently supported. I have a plan to add this to retroarch a bit later, the way it would work is you would put “[game_name]_Background.png” as a texture name for instance then [game_name] would be replaced by the game name that retroarch knows about. It would also probably support [core_name] as well.
Ha ha, those sound like some famous last words!!!
I do think it won’t be really hard to implement although it will be some work because we have to do things like check if the textures are actually there and if not fall back to a default texture.
Anyway you can be confident it’s on my list of TODOs