I know this is like a four year old thread but it still looks open, so I'll share some thoughts and see if they post.
I found an interesting article a while back on the topic of stenography and other tools used in virtual environments like SL for the purpose of, among other things, allowing dissidents to communicate without as much fear or harassment or persecution by their governments (e.g., Iran, Communist China, Russia, Venezuela), or for espionage, terrorism or other nefarious activity. Like all tools, it can be used for good or evil. Anyway, the article (from the US Defense Technical Information Center) is a bit dated (2008) but covers some interesting issues. Page 13 discusses stenography specifically. http://www.dtic.mil/dtic/tr/fulltext/u2/a488794.pdf
Programs like Adobe Fireworks (no longer available) used to the .png file format to layer images, which made it a popular stenography tool because many people would assume those images were flat. The thing with pngs uploaded to SL, however, is that they do actually get flattened, so hidden layers become cast in stone, as anyone who's uploaded a texture for a piece of clothing and then tried to share it with a friend to edit would find.