I guess I'm confused. Are these images being masturbated to being viewed by you or her? If it's you don't use those images. If it's her it's her fantasy and you're approaching jealousy of you're trying to tell her what she can and cannot look at and be turned on by. This is bad.

Fantasizing about anything is pretty much OK as long as they remain fantasies. There are of course fantasies that should never ever be acted on.