GraphicConverter gives even more options for slide shows...
Then you can use iCal to execute a script to run GC or slideshow at a certain time.
A bit about scripting GC...
A solution would be in two parts, the first part would be a script to run an application and start a slideshow. As suggested by BDAqua you could use GraphicConverter to do a slideshow of picture files e.g. JPEGs, or you could use Preview, or for a more sophisticated slideshow you could use PowerPoint or Keynote.
The second part would be to set the Mac to turn off and on automatically at specific times. This is done in Energy Saver settings in System Preferences. There is a Schedule button in the bottom right corner.