To customize the system.img image, you either need to extract the files or mount the image on your PC. Let’s mount the image.

• Copy the system.img file from your SD card to your PC.
• mkdir stock-system
• sudo mount -o loop system.img stock-system
• genext2fs -d stock-system -b 300000 -a system.ext2
• mkdir temp
• sudo mount -o loop system.ext2 temp/

—Make Changes—

• sudo mkfs.cramfs temp/ system.1.5.img
• sudo umount system_image
• sudo umount temp

source: http://webcache.googleusercontent.com/search?q=cache:LTOTe0EhO8cJ:wiki.xda-developers.com/index.php%3Fpagename%3DAndroid%2520system.img%2520Customization+how+to+mount+system.img&cd=1&hl=en&ct=clnk&gl=us