Starry Night by Edvard Munch.
Faun and his Family by Lucas Cranach -- I think this one might contain mummy black for the black portions.
Still Life with Fruit by Jan van Huysum -- it's hard to see in this miniature version, but there are insects crawling all over the fruit and flowers in this picture.
Christ on the Cross by El Greco.
Astronomer by Candlelight by Gerrit Dou -- this thing is amazingly small for how detailed it is.
I've seen some other fantastic museums around the world lately as well. Stay tuned for tales of the Louvre and the Victoria and Albert!