Wanna Make My First Mod, Replacer For Gas Dispenser

I've never made a mod before, but I want to try to make one that replaces the Gas Dispenser for Tag Team with the Medical Syringe from Killing Floor 1.
From what I can tell, I'd need to override the name and the image(s), but I don't know where to start.
I've successfully replacered the gas dispenser.
Focusing on the images, what filters should I use (in GIMP) to replicate the style of the images?