take the picture of the gun you edited
and open rez inject
chose the rez file (like if you are editing a sniper you would choose guns_mp_sniper or something like that .
just remember you have to chose the correct rez file cuz some are for the gun another one is for the texture and the last iss for sound
then choose the picture of the gun you have and click inject
then remove the original rez file (like guns_mp (something like that)
and rename the end of the new rez file the injector had made (its called the same thing but you see .reznew instead of .rez so rename the reznew to .rez
hope it works and if it doesnt pm me and ill send you a link for a better tutorial