David Barsalou has scoured the comic word to locate the source images for many of Roy Lichtenstein's paintings, and compares the original images side by side with Lichtenstein's paitings. presented them both side-by-side. Here's a sampling (and perhaps my favorite painting):
For the record, I think this exercise shows how accomplished Lichtenstein was since without fail, his paintings are huge improvements over the original images. I was lucky enough to see the above at the High Museum, and it is absolutely incredible.
NP: "Avery Island; April 1st" - Neutral Milk Hotel