How do I open the photo booth pictures from the screen?

If the event's display shows a guest QR code, scan it with your phone. It opens a booth pass where you can see the booth pictures guests sent to the screen, react to them, and keep one: open a picture and press Keep to save it to your phone. On an iPhone or iPad, Keep opens the share sheet; choose Save Image there to put it in Photos. No account is needed. The code can be on the screen before the booth has taken anything; the page then fills as pictures are taken. At an event running awards, the same page shows them above the pictures, with a button to vote: see How do I nominate and vote for awards?

The pass does not open the whole event, its guest list or its ordinary photo album, and a booth picture a guest kept off the screen is not in it; invited guests find those in the event's photo album. If you were invited, the sign-in at the foot of the pass takes you to the event's photo album with the address your invitation went to. If that address is not on the guest list, you will be offered the RSVP when the host has opened it to anyone with the link, and otherwise told the page is for guests.

At an event with a photo album and no booth or awards, the code on the screen is labelled Add photos and there is no pass to look at. If you are signed in as a guest it opens the event's photo album, where you can add your own pictures. Otherwise it asks you to sign in with the address your invitation went to, because the album is for guests on the list.

If the code is absent or no longer works, ask the host. They control the display and its guest access.