Need help with unrooting my device

hey all, and thanks in advance for your help.

ive rooted my galaxy s3 and install the cyanogenmod 10.1 on it. i want now to unroot my device and add the original os, my phone is intentional, (bought it from a computer store and not from celular company), anyway what i need is help where i can find the international stock image, and how should i do it ? im really rookie with all those things, so tutorials for beginners wil help me lol

thanks!