With Picasa 2's collage feature, you can simulate multiple exposures, create a grid of pictures, or make a collage of a pile of pictures.

To make a collage:

1. Select two or more pictures and click the Collage button.

2. Choose from each of the options below:

Type:

Picture Pile: Arranges your pictures in a random pile

Picture Grid: Crops each picture to a square and forms a grid

Contact Sheet: Creates a contact sheet style picture with thumbnails

Multi-Exposure: Layers pictures to replicate multiple exposures on film  

Options:

Use Background Picture: Uses a selected picture as background image

White Background: Displays a white background

Gray Background: Displays a gray background

Black Background: Displays a black background

Location:

Save As Desktop Picture: Saves as your desktop background

Current Folder: Saves to the folder containing the source pictures

Screensaver Picture Folder: Adds the collage to your Picasa screensaver

Choose a Folder: Allows you to choose where to save the collage

Click on the preview picture to randomize the picture order for the Picture Pile and Picture Grid collage types.

Shift-click on the preview picture to rearrange the layout for the Picture Pile

When you are happy with your collage, click create.  The file will be saved to the location you specify.