Three Worlds, M.C. Escher
Lithograph, roughly 14×10 inches (36x25cm). Image on Wikiart, larger here.
While it’s not one of Escher’s more obvious brain twisting visual conundrums, it’s a teaser nonetheless — also beautiful, subtle, and one of my favorites.
In addition to the thought provoking subject, superb drawing and beautifully handled reflection and surface perspective, I love the way the composition transitions graphically from dark against light at the top to light against dark at the bottom.