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.