Figured out how to create stereograms "manually" in GIMP. No plugins, special filters, anything....It's a fairly simple process, although a bit repetitive. It's not perfect as stereogram programs have algorithms to minimize "ghosting" (aka "sliver" effects), which can be unsightly, but it's really cool to do it manually. Once I get the technique refined, I'll prepare a tutorial.
So, what do you see hidden in the texture? Click on the thumbnail to see a fullsize image.