I just added a package on AUR which enables using Picasa as a Photo Viewer. Got the inspiration from here.
If you already have
yaourt installed, then installing the application is a breeze.
yaourt -S picasa-photo-viewer
Sample installation log is shown in the following screenshot
Ubuntu Tweak is a good utility to change the default photo viewer to Picasa, looking for a ‘PicasaPhotoViewer’ application in the list.