In Mac OS X, you can display a picture on your Desktop, and with iPhoto, putting one (or more, in rotation) of your photos on your Desktop is a matter of just clicking a button.
To set the Desktop picture:
Select one or more photos, and click the Desktop button or choose Desktop from the Share menu (Figure 5.21).
Figure 5.21. Click the Desktop button to set the selected photo as your Desktop picture.
If you selected only one photo, iPhoto immediately changes the picture on your Desktop (Figure 5.22).
Figure 5.22. With a single click, you can put the photo you have selected in iPhoto on your Desktop.
If you selected multiple photos, iPhoto opens the Desktop & Screen Saver preference pane (Figure 5.23).
Figure 5.23. In the Desktop pane of your Desktop & Screen Saver preferences, configure how you want your Desktop pictures to appear, and how often they should rotate.
If you selected multiple photos, choose how you want the images to appear on the Desktop.
You can also display the photos in an iPhoto album on your Desktop by selecting the album in the folder list in the Desktop & Screen Saver preference pane's Desktop tab. (Note that rotation among photos doesn't work within albums.)
If the picture is in landscape orientation, iPhoto scales the photo to make it fit.
If the photo is in portrait orientation, iPhoto takes a landscape chunk out of the middle to display on the Desktop. Stick with photos in landscape orientation, or crop them appropriately first.
iPhoto can put a picture on only one monitor. To put a picture on the second monitor, open the Desktop & Screen Saver preference pane and select the iPhoto Selection folder to rotate through the same photos on the second monitor.