Presuming you have a high enough post count to add a picture, you can size it down to 800 x 600 in almost any Paint type program (or use Windows built in Paint program) before your add it to your post.
Pictures always help things sell . . . as does including a price in your listing. Even if you add "or best reasonable offer", adding a price helps people decide to pick up a bargain and shows that you are serious, and reasonable, about selling the gun.
Best of luck!
It would be so nice if something made sense for a change.
-- Alice in Lewis Carroll's Alice's Adventures in Wonderland