Why can't I add more photos to my project?
Apple allows you to decide if you'd like to permit apps to access all of your photos or only a few you select when you first grant this permission. If you only allow access to select photos, the SimplePrints app will not show any other photos you have stored on your device or iCloud.
If you want to add more photos, open Settings on your device, scroll down to "SimplePrints", tap "Photos", and ensure you have "All Photos" selected. When you return to the photo picker in the SimplePrints app, you should see all your photos available for selection.