Today, Google released Jelly Bean factory images for several Nexus devices, including Galaxy Nexus, Nexus S and Nexus 7.
These factory images are useful for those who prefer to mod their devices and need to get back to a stock version of the operating system. The latest Jelly Bean factory images are available for download here.
Handset Nexus ROM that was provided by the Google Android 4.1.1 HSPA + is the Galaxy Nexus, Nexus Nexus S up to 7 tablets. Unfortunately for now Jelly Bean ROM for Nexus CDMA handsets are not yet available.
With this ROM developers can play around to modify the existing ROM with more calm. This is because if they do something that is not good, they will be able to do flashing back by using the original ROM from Google that.