Hi there!
If you can't see your photos in the Select Images screen:
Did you allow Picture Postie access to all of your photos? You may have accidentally only allowed access to a limited number of photos. To check
Go the the Settings app on your phone
Search for Picture Postie at the top
Open the Settings for Picture Postie
Ensure Picture Postie has access to All Photos
See this video for a simple demonstration on how to allow access to your photos
2. Still can't see your photos? When you open your photos in the Picture Postie App, they will be shown in date order. If you have added old photos to your camera roll, while they might show at the top in your Photo app, they will appear in date order in the Picture Postie app. This allows your photos to be printed in chronological order, which is helpful when sorting through them.
This only applies if the photos you have added to your camera roll have date information attached to the file, if not, they will show at the time you added them to your Photos app.