Mac Gaming Mods

So I want to make a weapon prop for myself and I have a picture from deviant art or w/e.
I want to print more than 1280x800, sadly when I click "print" and show me the review and it tries to resize to fit my paper.
(Of course I must always check before printing).
Can anyone give me an application that can divided part of my picture?

Have you tried something like Gimp or Pixelmator?

I have Gimp, I don't know how to make it work