There is an option in pimp my gun to save the image. If you use that, it will save just the gun, and not the entire window.
A few of my creations:
An MP5 bullpup. Would probably use the F2000 type shell eject. I believe that's a 20 inch barrel for comparison.
Modified M14/M15. PRS stock, bipod, monopod and a palm rest.
A loaded AR. Although really it's a 416.
SR25 sniper rifle, with some G36 features, and a PRS stock.
And not really my design, but the local field owner was selling these a while back: