iMessage tells me address is not registered

Hi

I have set up a new iPad for my mother, we want to share my photos from my iPad to hers. both from the photos app and lightroom. at the moment i am working on the photos app, if any info is going for the other thats great but i am happy to make progress anywhere i can.

I have selected some images and tried to share them it is not a mac e.mail but it is working fine on the iPad. It appears to be sending it to iMessage, but iMessage is not happy with the deal. Photos app says the images have shared.

Does this make sense to anyone, i think i amore confused now then when stated.