if you are using a MacBook, you can use the following steps to mirror your screen to an Apple TV:
Make sure your MacBook is on and connected to the same Wi-Fi network as your Apple TV.
Click on the Apple logo in the top left corner of your screen and select "System Preferences."
In the System Preferences window, select "Displays."
Click on the "AirPlay Display" drop-down menu and select your Apple TV.
Check the box next to "Show mirroring options in the menu bar when available."
In the menu bar at the top of your screen, click on the AirPlay icon (it looks like a rectangle with an upward-pointing triangle).
Select your Apple TV from the list of available devices.
Your MacBook screen should now be mirrored to your TV.
Open the Photo Wall on your laptop's browser, and set it to full screen. You should now see the slideshow on your TV!
You can follow this Youtube video guide for additional info:
Pro Tip - Set the browser to full screen to hide the browser frame so it takes the entire space available on the display. It's also suggested to connect the laptop to a power adapter so the battery won't die during the event.