04/17/2012 AT 5:25 AM ET
Arriving at the Ciroc vodka-sponsored soiree with a group of pals, the actor excitedly jumped at the opportunity to contribute to the Paul Frank mural that featured many intricate designs by other various partygoers. He painted a portion of the wall, which will be donated to GenArt.
“He selected what he called the “sexiest” part of the mural to sign,” an onlooker tells PEOPLE. “It featured a woman’s leg as the guitar strings.”