You fail to describe what problem you're trying to solve (apart from "I have nude pictures of my girl on it. *blush*") and from whom you want to hide your home made pr0n. Also, keep in mind that the N900 wasn't designed to be a spook tool. SMS messages are stored in a data base and to hide some messages, you'll need to, as far as I know, hide them all. Images are indexed for a reason so trying to hide them goes against the design.