Which pretty much covers off the last part of what I said "so unless there is a big problem caused by them they don't really need updating."
Generally though themes are rarely updated and 9/10 it's just to tweak the display of something for one browser or another or add in a couple of design changes or occasionally a feature, as long as you read the release notes and be sure you're not missing anything important it's okay. There's never a foolproof way of circumventing people grabbing your images so there's no point stressing over lightbox/fancybox type exploits, if you want to prevent copying the only was is watermarking.
