You might want to take a look at this answer or, even better, at Android Folder Hierarchy, which most likely gives you the necessary details.
Using that information, your best approach to backup via ADB might be:
# find the device for the partition
mount | grep system
# from the output of the above (data from your screenshot), now back it up
# using "dd":
dd if=/dev/block/mmcblk0p12 of=/mnt/sdcard2/system.img bs=1M
if is the device to back up,
of the target (I just placed the image file in the root of your sdcard, you might wish to adjust that). Take care not to mix up the two, or you might destroy your system! (this is why
dd sometimes is backronymed "data destroyer").
bs defines how much data to read/write in a single operation, my example uses 1 MB; you might play with this and e.g. use "8M" to speed things up, but do not set this too high.
Above example just handles the
/system partition. Repeat it similarly for any of the other ones listed in the linked question which you want to have backed up.