Can't backup Stock ROM with clockworkmod recovery...?

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http://forum.xda-developers.com – So please keep in mind that i'm a total noob about this kind of thing. So my phone is already rooted, and I have clockwork recovery installed and I was trying to backup my stock ROM, but it won't back up. So I seen over the internet that I could backup from my recovery mode, but there is no option. Is there anything I am missing? Can anyone tell me please? I heard about something called ODIN, but I have no idea what it is or what to install it. (HowTos)