see:
https://forum.beagleboard.org/t/create-a-full-image-to-redeploy/34628
Summary:
sudo dd if=/dev/mmcblk1 bs=1M of=<my_image> bs=1M status=progress
should work in most cases, otherwise use the beagle_flasher tool.
Note: if you want the resulting image to be a ‘flasher’ image, I believe you’ll need to mount the image then edit it’s uEnv.txt file.