This article explains how to replay a Snap in Snapchat using the Snapchat Replay feature, in case you missed the photo or video and weren't able to view it properly.
How to View Snaps MULTIPLE Times on Snapchat [WITHOUT Them Knowing] [View a Snap More Than Once]
Here's how to replay a snap:
After you view the snap, make sure it stays on your Friends screen.
After you view your friend’s snap that you want to replay, you should see two pieces of text flashing back and forth under their name. One is labeled Tap to Chat. It will switch to Press and Hold to Replay, and then back again.