To set this up on your Mac, open iTunes and pick File> Home Sharing> Turn On Home Sharing (if it’s not already on).
Then you’ll need to enter your Apple ID info in the subsequent box and click “Turn On Home Sharing.”
When you’ve done that, go back to File> Home Sharing and select “Choose Photos to Share with Apple TV.”
You’ll then be taken to a dialog box to pick which images you’d like to use. You can also decide whether to share from iPhoto or from a folder, for example, and if you don’t wanna send your entire library as I’ve done below, choose “Selected albums, Events, and Faces” instead and go nuts with checking the appropriate boxes.
When you’re done, click Apply, and head on over to your Apple TV to configure Home Sharing there. First, go to the big “Computers” button on the Home screen…
…and choose “Turn On Home Sharing.” Enter the same Apple ID that you used on your Mac, and you should see your content appear, ready to be viewed. Cool!
An important note here is that, in order for your images to be visible on your Apple TV, your Mac can’t be asleep, and iTunes must be open. Since recent models of the Apple TV don’t actually store any photos on an internal drive, those files will be streamed across your home network instead. That’s a bit of a pain, but hey, it’s a small price to pay for being able to show several thousand vacation photos to your houseguests. That’s the way to be a good host, right?