Most, if not all, world-famous paintings cannot be photographed and are protected by eagle-eyed museum personnel ready to whisk you away from the "scene of the crime" if you insist on clicking your cam.
In the Louvre, for example, my friend Archie and I tried in vain to take a pic of the Mona Lisa. It's probably the most protected masterpiece there is, as plainclothes museum staff from out of nowhere would just tap your back or cover your cam to prevent you from taking a shot. So, basically, what I'm trying to say is, I STILL DON'T HAVE A MONA LISA PIC WITH ME, Aaarrgh!
Michelangelo's Last Judgment in the Sistine Chapel, the Vatican.
Sandro Botticelli's The Birth of Venus in the Uffizi Museum, Florence, Italy.