I'm only experienced in FS2004 repaints myself. Are you looking for advice to get started on your own repaints? Or looking for an artist to paint something for you?
I have only had x-plane 9 since last Christmas and I am trying my hand at repainting some of the aircraft on it my first attempt was a Sabena Boeing 737-200 which looked ok any advise would be great
I notice the wings have various panel and control surface marks, while the fuselage does not. Perhaps you can find a blank skin that has these for the fuselage? Once you have those lines and other details, you can use them as a separate later above your paint, and work with various tools (depending on your graphics software) to smoothly integrate them into your livery.
Also, I see a faint black seam along the top of the fuselage. This is easily fixed by extending your paint a bit higher on each side of the fuselage texture.
If your aircraft's repaint is already in a format that you can work with (such as BMP), then open it in GIMP, and it should appear as the background layer.
From there, just add a new layer to the stack, and paste in your markings to that layer. Position them and save the file.
Note:
If your repaint file is in a format that requires pre-processing (such as those with alpha channels), then you'll need to use what ever tools do that job both before you edit in GIMP, and after you export the edited work.
Unfortunately I'm not familiar with the X-plane texture formats, so I can't give a more precise help related to them.