I found some great examples of dollar bill art on uproxx.com
"At some of my newest favorite sites like Collection Studio, Visboo, Defaced, Slummo, If It's Hip It's Here, Deviant Art, The Heist, Greedmont Park and Heated Fornication, I found more than just dollar bills with messages written on them. I found artwork. Granted, it's artwork that is considered a federal crime and in no way do I condone the desecration of American cash, but let's say for entertainment's sake that it is pretty cool. On that C-note, enjoy this gallery of defamed currency."
The following are the ones I really like on uproxx
(This and a few other pieces are from a collection by tattoo artist Scott Campbell, who carves designs into stacks of cash. It's a little bit more impressive than simply drawing Spider-Man over George Washington's face.)