[G4] Tiger Desktop Changing

[shopdog]

All:

I'm running Tiger 10.4.2.  It used to be (I guess before Tiger?) that  
I could (in the System Preferences "Desktop & Screensaver" preference  
panel) set the desktop to show pictures from an iPhoto album, and  
change the picture every few minutes (to the next picture in that  
album).  Now, although I can still choose an iPhoto album, the option  
to have it change the desktop every few minutes is greyed out.  It is  
not greyed out if I choose a picture folder on my hard drive, or if I  
choose an apple desktop pattern.

Does anyone know why the option is greyed out?

shopdog