If these are photos you had on another device, you will have to get them from that device. If on a stolen or missing device, they are gone. Now if, however, the photos are either on a google account and you want to get them, or you accidentally deleted them from the phone and are trying to get them back, instructions/info on both are below.
To try and get photos back from a Google account...
1. Open the "Google Photos" app.
2. Tap the photo you want to save to your phone.
3. Now tap the "Options" menu.
4. And finally tap "Restore".
5. Repeat for each image.
Or, to get them all at once...
1. Log into the google account associated with the app.
2. Download all the images to an empty folder on your computer.
3. Connect the phone to the...