> There is no "Open with" Photos. Not even possible for Terminal.

Can you please file a bug against gnome-photos, in the GNOME Bugzilla,
about this?

 
I think it isn't a bug, it is just the way that Gnome Photos work. More like image indexer rather image viewer.
But I reported in any case
https://bugzilla.gnome.org/show_bug.cgi?id=758705