Continuing with their tradition of striving to offer up the newest version of the Android OS as quickly as possible, Google just recently posted the full factory images for Jelly Bean on all of their latest Nexus devices. At the source link below you can find factory images for Jelly Bean version 4.2.1 for each of the Nexus 4, the Nexus 7 and the Nexus 10. They also included the factory image for the Galaxy Nexus (GSM Version). Of course, this is super-handy if you are tinkering around and need to restore your device for some reason.
Here's the link: https://developers.google.com/android/nexus/images