After upstaging the fashion royalty at the Costume Institute gala, Katy Perry (above) will headline the CW’s upfronts at Madison Square Garden tomorrow. The gorgeous singer will perform her new single, “California Gurls,” which has been selected as the network’s official theme song for the summer. Also expected: “Gossip Girl” boy Chace Crawford, “Vampire Diaries” stars Ian Somerhalder, Paul Wesley and Nina Dobrev, “90210” beauty AnnaLynne McCord and “Superman” star Tom Welling.