Click on the line tool (
). Now go to the top toolbar, and click on the Custom Shape Tool (
).

Now go to the drop down box next to the box that shows the shape. Choose a shape you like and apply it to the picture. Wa-la!! You're done!! You now can refer to my other tutorials to learn how to apply other effects to the shape like making it 3D like or even adding text, or pasting the shape to an existing picture. =)