Collage is one of Picasa's most interesting features. You can use it to combine multiple images into a picture pile (Polaroid-style photos) or a multiple exposure (one picture superimposed over another), for example.
Picture Pile example
Select the pictures from which you want to create a collage, adding them to the Picture Tray.
Choose a collage style from the Type pop-up menu: Picture Pile, Picture Grid, Contact Sheet, or Multi-Exposure.
Choose a background color or image from the Options pop-up menu.
Optional: To specify where to store the resulting collage (different from the proposed location), choose a folder from the Location pop-up menu.
To pick a folder other than those listed in the Location menu, choose Choose a Folder and then click the Choose button.
Click Create. The collage is created and stored in the specified location on disk.