Layers in Gimp we are not in Poser yet.  After you make your white marks, you may have to blur them slightly.  So do not close Gimp, you will need to go back and forth several times to get the effect you want.

We'll get to the Poser side and the texture node that will contain your .jpg soon.  As soon as you have your lines done, I want to see it.  Do a print screen and post here.  You should be using the Alyson texture map, it's called a UV map.  Since I have mine already, I can see if everything is positioned correctly.