In order to add photos via the + button, the app requires permission to access the photos on your device. If you have denied access when first asked, please follow these steps:
- Open the Settings app on your iPhone or iPad.
- Tap on Privacy.
- Tap on Photos.
- Find Print to Size in the list and tap on it. Tap the “Read and Write” option to allow photo access by the app.
Note that the + button isn’t the only way to add photos. You can also use the method described here