Make sure your Picasa account is added under Accounts & Sync. I have a Razr Maxx on Ice Cream Sandwich, so I don't know exactly how the Settings menu looks on Jellybean; however, there ought to be a selection for Accounts & Sync. If you don't see your Picasa account listed there, then press "Add Account," scroll down and select Picasa, and then enter your login info and password at the prompt. Then when you sync, the photos should start appearing in the main My Gallery app for the Nexus 7.