Hi Jeroen!
You need the 'GP3 Jammer' program. I think its on the second page of editors/utilities.
First load up the jam you want to replace, in this case the original cockpit jam form GP3jamsh.
Then open your 'import' your replacing bitmap (your cockpit)
Now it is a jam file, so press 'save jam'
Now your file has replaced the original. Remember its always a good idea to make a backup of the original.
Chris J