Thank you so much for all your help, will try it out!

I've enabled the icloud photo library, and no 'camera roll' in sight. Is there no way for your app to find these images?

Yes, I took the pictures with the phone, some of them are from a travel where I didn't bring my ipad. I've tried to switching off the app and the phone and switch it back on, but still images are missing.

Took a new photo, and found it immediately.

A mystery!