As for the temp-tattoos, what about Sharpie Markers? Kinda depends on how your skin reacts to 'em-- for me, a mark stays on for several days; for others (Icka, for instance) it'll fade very quickly. Worth a shot, though. Woad, just as a caution (and yes, I've tried it) is interesting, but it's less reliable than a lot of body-paints.

Paintings... what about mermaids? Figleafed but not clothed (I hate the stereotypical bikini-top mermaids), maybe sunning on the rocks or trying to seduce sailors? Or better yet, being seduced by sailors? XD Turnabout's fair play, after all. Or if you want to play a bit, do a scene from Snow White & The Seven Dwarves, only do it LotR style-- make the dwarves look like DWARVES and Snow White as an elvish maiden in a glass coffin.