Acquiring the 40 famous paintings for 40 Most Mentioned was the hard part. Slicing and splicing was simple.
Creation of 40 Most Mentioned – Mona Lisa is based on Visitors of Louvre in front of Mona Lisa by Edal Anton Lefterov, available at http://commons.wikimedia.org/wiki/File:Mona-lisa_in_the_Louvre.jpg, and is likewise licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license (CC BY-SA 3.0).
Disclaimer: No physical paintings were harmed in the production of 40 Most Mentioned. The Mona Lisa was intact when I visited the Louvre in July 2013. If any of the paintings have been stolen or damaged, it wasn’t me.