Now you’ll be presented with the Photo Viewer Configuration window. Select to Use Picasa Photo Viewer as the default viewer , then select the file types you’d like to viewer with the Viewer or just click Select All . Don’t worry; if you don’t like the new Viewer, you can always revert back to your previous settings.
You can now look around the Picasa program, or just head back to Windows and continue with your work. Next time you want to preview a picture, simply double-click the photo or click the Preview button in Windows Explorer.
Your screen will dim, and the picture will zoom out and display in a full-screen overview. No window chrome, just your picture and your desktop behind. The preview loads almost instantly, and looks sharp! Press the left or right arrows on your keyboard to go to the next images in the folder, or scroll in and out of the picture by scrolling with your mouse wheel. Close the preview by pressing the Esc key on your keyboard, or click the X in the top right corner. The preview works beautifully, with zoom in and out animations when opening and closing a preview.