National Gallery of Art, Washington, D.C.
Albrecht Dürer was a German painter and engraver. It's not entirely clear to me why he decided to include a monkey in this engraving of the Virgin and Child, but I'm glad he did, because as we all know, there is nothing that cannot be improved by adding a monkey. (Seriously.) I'm actually cheating a little bit with this one; the National Gallery's copy of this engraving is not part of their online collection, so I'm borrowing this image from the British Museum. But hey, it's a print. Obviously, the National Gallery's copy looks just like this one.
And now for some pointless nostalgia. The third grade hallway of my elementary school was decorated with reproductions of famous paintings, including Dürer's famous watercolor painting of a young hare. As an elementary school student looking at that painting, I assumed that the AD at the bottom of the painting, below the date 1502, stood for anno domini. It was quite a revelation some years later when I recognized the distinctive monogram on another work by Dürer and realized the mistake I'd made.