Best way to forge an image from static images(Please answer...has cookies!!!!)

Ok So I have static Images of a sprites Arm which I want to put a sword in its hand(dynamic image), I hear alot about reference pixels how do I set one up? I imagine that I mark an image with a RBG value marked for reference and then find that pixel in the Image and then paint my sword at that pixel? But how do I go about doing so, and is this the right approach?
ALL help is appreciated, thanks in advance.