You can create a full-screen slideshow for your photos with a single click in both the Picasa software and Picasa Web Albums.
Picasa
For any folder or album in Picasa, just click the 
Play button in the folder or album header. To create a slideshow using photos from multiple folders and albums, first create a new album using these photos.
Use the controls at the bottom of the slideshow to change your slide transition, adjust slide duration, include captions, rotate and magnify images or set your slideshow to music by adding supported audio file types. You can also set your slideshow to play continuously: Click Tools > Options (Windows) or Picasa > Preferences (Mac). On the Slideshow tab, select 'Loop slideshow' to play your slideshow over and over.
You can exit the slideshow by clicking the Exit button at the bottom of the slideshow, or by pressing the Esc key.
Picasa Web Albums
Picasa Web Albums also lets you view a full-screen slideshow with a click:
When viewing an entire album click the 
Slideshow button. Use the controls at the bottom of the slideshow to adjust slide duration, play/pause, and the inclusion of captions.
When viewing a single photo click the Full Screen button. Identical slideshow controls are available, however the slideshow is paused to provide a full screen view of the specific photo.
You can exist the full-screen view by clicking the X button or by pressing the Esc key.