Picasa 3 ships with a new Photo Viewer, which’s designed to be a better, faster image viewer. A quick double-click on any photo will give you a beautiful full-screen view (with Picasa edits applied) and puts a select few options at your fingertips: you can quickly rotate an image, star it, upload it to your Picasa Web Albums drop box, or play a slideshow with other photos from the same folder.
Also, Tap the scroll wheel on your mouse to discover how nice it is to have a fast, smooth zoom at hand when reviewing your photos, or try the 'More Options' button to bounce an image to your printer, blog, or favorite photo editor.
A note on installation: You can add the photo viewer during the Picasa 3 install -- it's easy to revert to your previous image viewer later, if you'd like. If you've already installed Picasa 3, and originally chose not to use the viewer, but now want to give it a try, just go to "Tools / Configure Photo Viewer" in Picasa.
Source:→ Google Photo Blog