By OpenWrt distro, do you mean you are building your own image from scratch? If so you are better off using the LEDE stable release as the old OpenWrt has not been updated for a few years now.

You can’t easily backup the image, but then there is no real need to do so if it is your own image, just include everything you need.

Post image data/config backup (excluding packages) can be done via Luci UI or the sysupgrade CLI.

The data to be backed up is controlled by the file /etc/sysupgrade.conf

Bare in mind that anything included in the image that is backed up then restored, will use at least double the amount of flash memory when restored (a consequence of the read only, compressed data in the image).