The mapping is pretty well done. It would look a lot times better with bit of simple shading. BTW shading in GIMP is easy. As for the way you have done it, some of the lines are blurred and there are a few gaps in the 3d bits on the side of the visor. I have to admit, when I first saw it, I thought it was an early stage WIP. This head has a fair bit of potential. If you did shading, cleaned it up and added detail on the back, it could look quite nice.