As you know, Snapchat photos delete themselves after creation, so that even Snapchat can't retrieve them once they're deleted. Our integration with Snapchat is for your saved Snaps. Any Snap that you save can be found in your camera roll.
Go to your Camera Roll folder in Lifeprint and tap the “Camera Roll” text. All of your folders will pop up. From there, open your Snapchat folder, where all of your saved Snaps are conveniently there for printing. Have fun!