Can anyone diagnose my problem with installing Roms?

My gnex is rooted running a 4.2.1 custom ROM. I want to install 4.2.2 but every time I try to install the zip file through ROM manager, it goes into recovery, starts the process and then, after what seems like a successful process, it boots back up to my regular home screen with nothing changed. I've never had this problem before. I just want to install stock image while staying rooted. I have the file downloaded and everything. Can someone please help?

edit I downloaded the stock image right from Google and (this is what shows up in downloads)[ http://www.imgur.com/TrKVgPU.jpeg] why is it not a zip file? I tried making it into a zip file and installing that as the ROM and it did not work either... Installation keeps aborting.