Booty call!

Kim Kardashian knows her strongest asset.

The curvaceous Keeping Up with the Kardashians star, 29, had a clear vision for the design of her eponymous Appetizer Mobile iPhone app, which includes her beauty tips, tweets and several animated screenshots of herself.

"Kim approved it all," says a source. In fact, she paid special attention to one specific body part: her famed derriere!

"Kim asked them to revise her avatar three or four times because it didn't look right to her."

Her final pick? "She went with the biggest butt of all the options."

The source adds that Kardashian "was happy with the app," now on iTunes.