Under the "Create" menu is a menu option called "Picture Collage".
If you select that item, you will be prompted with a settings menu on the left with a drop down list. In the list you want to select "Multiple Exposure: Superimpose pictures over one another".
Click on the "Clips" tab, and select the photos you wish to superimpose. Click the + sign (plus) to add them to the collage.
Then go back to the settings tab, and click "create collage".
The editing options in this function are fairly limited, but you can do a simple superimpose with it if you first create clipped photos, etc., so that when they are superimposed the alignment, etc., are appropriate for the output.
You can also use the "Picture Pile" option within the collage menus to superimpose/overlay one photo on top of another, if appropriately sized and cropped.
If your looking for a very simple superimpose function in Picasa, this would be it. Anything more powerful would require a different tool, like the type mentioned by Nimman's response. Thanks
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