[This article, I believe, tackles general interest topic so I lifted it from my personal blog.]
How do we produce these effects? Incredibly easy. Moments after you install Picasa, it will find all the pictures, and some supported video files too, in your hard drives and immediate network, including long lost pictures whose locations you have lost track of long ago. The pictures shall be organized in folders for your effortless access. In Picasa’s application interface, CTRL + select the pictures you like to include then click Picture Collage from Picasa’s Create menu. You will immediately see the preview of your collage. When satisfied with your creation, save the composite picture as graphic file.
You may select from any of the four options for your final collage output: picture pile, picture grid, contact sheet and multi-exposure. You may choose white, gray or black for background color of your collage, or you may set the farthest picture as your background graphic.
Download your free Picasa here to explore its other features. Very soon you will start posting professional-looking graphics to spruce up your blog.