Use Photos from iPhoto !

Use your Mac Desktop as a large digital frame of sorts. You can show favorite pictures from albums you've created in iPhoto, and have them change every so often, too.

Open System Preferences (in the Apple menu). Click Desktop & Screen Saver (in the first row). This will bring-up settings for Desktop wallpaper and Screen Saver. Then, click the Desktop tab.

If the left column of the settings windows, you'll see a number of folders from which you can choose images to be displayed on the Mac Desktop.

Toward the bottom of the list, iPhoto Albums is listed. Click the triangle to view the albums you've created in iPhoto. Select one of the albums. Your Desktop will change instantly, displaying a photo from the album.
Now, in the same window, check the "Change picture" setting and choose when or how often you'd like the Desktop wallpaper to change. For example: every 30 minutes. You can click the "Random order" setting to have the next Desktop picture randomly selected from the iPhoto album.

One final finesse. In the settings window, above the thumbnail photos, you can specify how you'd like the photo to display on the Desktop: Fit to Screen, Fill Screen, Stretch to Fill Screen, Center and Tile. Play with the options and select what you like best.