mount (empty) partition, mount image file as loopback (mount -o loop <imagefile>, if I recall correctly), then cp -a content of the latter to the former.

then, You need to modify .chroot file, pointing it to partition, instead of image file (refer to Easy Debian wiki for instructions about that).

/Estel

// edit

As for backing it up to image file, You just revert whole process - mount Your file (clear files there, if it's not empty), cp -a everything from partition into file.