You will have to download adt-bundle-linux
Extract it to some place(assuming home)
Change is permissions vi terminal ( chmod +x home/adt-bundle-linux/sdk/platform-tools/adb )
Then cd to that folder
Type adb shell
Then dd if=/dev/block of=/mnt/sdcard (i assume you know the dd command and block)
Done! The system.img file will be present at sdcard!
And dont forget to hit thanks!
if I've helped