Childhood Friends, Karl Spitzweg
Link is to zoomable version on Google Art Project, downloadable file on Wikimedia Commons, original is in the Städtische Galerie im Lenbachhaus, Munich.
I love the illustrative qualities in the piece — the theatrical lighting, the narrative element and the caricaturish portrayal of the old friends, shown in an apparent reunion after one has gone off to see the world, and the other has stayed at home.
The way Spitzweg has used value contrasts — dark against light and light against dark — along with atmospheric perspective and color accents, to guide your eye is particularly nice.