Does Anbox need a specific Android image for every device?
No. Anbox does not need a specific Android image for every device as it's is especially build in a device independent way. Anbox will ship with a single Android image per architecture (amd64, armhf, arm64) and no device specifc modifications are required.
I attempted to try it on the devboard, it looks like it will require https://snapcraft.io/ for installation, there is a package for stretch, but not jessie, building it from source brought me into a rabbit hole of dealing with golang... so, I think I'll need to try out the Stretch image soon then.