Factory images are those files which can save your device should you run into trouble. After all, these are Nexus devices, and power users will likely be flashing all sorts of custom ROMs. You can use factory images to restore your Nexus device’s original factory firmware.
Following the source link below you will be able to grab the Jelly Bean factory images for the Samsung-made Google Nexus S (soju and sojua versions), Samsung-made Google Galaxy Nexus (yakju and takju variants) and the Asus-made Google Nexus 7 tablet (nakasi). Sadly, they’re not available for the Korean (sojuk), Sprint (sojus) Nexus S flavors and the Verizon Galaxy Nexus. Follow the source link below to grab them.