My shared photos can be found on Google Photos. It's not the perfect place to keep them, but it's pretty good.
You can access Google Photos through a web browser or use their app on your phone. The will alert you when new stuff is shared.
The Google Photos app is pre-installed on Android phones and may already be on your home screen, otherwise it is a free download for Apple/iOS.
Using a web browser?
Just open this link 👉 https://photos.google.com/sharing. You'll see any photos that have been shared with you, as well as those you may have shared with others.
Using the Google Photos app on your phone?
The Sharing button can be found under the menu icon on the top left. Simples.