This is for all who wants to unpack and modify the original system.img that you can flash using recovery.
system.img (what you get from the google factory images for example) represents a sparsed ext4 loop mounted file system.
It is mounted into /system of your device. Note that this tutorial is for ext4 file system, you may have system image which is yaffs2 for example.
the way it is mounted on Galaxy Nexus:
“/dev/block/platform/omap/omap_hsmmc.0/by-name/system /system ext4 ro,relatime,barrier=1,data=ordered 0 0″