@CiberSheep Thank you - I've finally (!) managed to move it to a writeable location :-). Just for info, the variation I used was
picker.activeTransfer.items[0].move("/home/phablet/.local/share/<app_name>/","picked")
as I'm only importing one picture.
A couple of extra questions - does anyone know if the above can be used to overwrite an existing file (the user may want to change the picture from time to time) or can the imported file (the one that we've moved) be deleted?
@CiberSheep Thanks again - your help has been invaluable

@AppLee
Updating d9 directly, as you suggested, worked a treat - I didn't realise it could be done so simply, but I'm happy that's the case :-).
Thank you so much for all your help - it's been invaluable.