From the Apple menu on your Mac, choose System Preferences and click the Desktop & Screen Saver icon. In the resulting window, click the Screen Saver tab, and then click one of the 14 photo-based themes in the left-hand list. When you do, a Source menu appears beneath the theme's preview; choose Photo Library (it'll take a minute to populate this menu) and a new pane appears at left listing your iPhoto goodies. Click an album to see what photos are inside and then click Choose. Back in the Desktop & Screen Saver window, you see a mini version of your new screensaver playing on the right.
Mac to Apple TV
If you own an Apple TV, you can use it to parade your photos on your TV in a mind-blowing manner. If you've got an iCloud account, sign into it on your Apple TV, and choose iCloud Photos or Photo Stream from the main menu. If you're rolling sans iCloud, fire up iTunes on your Mac and choose File> Home Sharing > Turn On Home Sharing (for iTunes 10.7 and earlier, choose Advanced > Turn On Home Sharing). Enter your Apple ID and password, choose File > Home Sharing > Choose Photos to Share with Apple TV.
The iTunes window changes to display a list of your albums, Events, and Faces albums. Make sure the "Share Photos from" checkbox is turned on, and then use the controls to share your photos — from iPhoto or those stashed in a folder on your hard drive. Turn on "Include videos" to include any iPhoto slideshows you've exported, and then click Apply. On the Apple TV, choose Settings > Computers > Turn On Home Sharing. Enter your Apple ID, and in the iTunes libraries shown (which could be from multiple Macs), pick what you want to see; as long as the Mac containing the shared library is turned on and iTunes is running, you're golden.
iPhone or iPad to Apple TV
With an iOS device and an AppleTV on the same wireless network, you're set to awe others by projecting your iOS device's display onto your TV. First, grab your iOS device and swipe upward from the bottom of your screen. Tap the AirPlay icon (circled) and from the resulting list, tap the Apple TV you want to project to, turn on Mirroring, and tap Done. Now your iOS device's screen appears on your TV. To play a slideshow, launch the Photos app, tap an album and, on an iPad, tap the Slideshow button that appears at upper right. On an iPhone, tap a photo in the album and then tap the share icon at lower left. Next, tap the Slideshow button that appears at the bottom (circled) and then choose a display device from the next screen. Click Start Slideshow to watch all the photos in the album play.
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