To upload photos press the camera icon in the middle of the bottom bar on your app. Here you'll have two possibilities:
Upload from library
The app will ask you for permission to access your photos and once granted your photo library will appear. You can select and upload as many photos as you want in a single batch.
Take photo within app
Click on the camera icon in the top half of your screen and the app will ask you for permission to use the camera. Once you confirm, your camera will appear and you can take a spontaneous snapshot!
You can upload multiple images directly from your computer. Let's say your event photographer sent you an entire folder of photos, then this is a comfortable way to upload them all at once.
To do this click on Gallery in your side menu.
On the succeeding page you can click on Upload and select all the images you'd like to share with the other guests.