I can't help you with your specific request, but a can make a suggestion.
If you scroll down on the main page you will find a forum called Tutorials/How To... and in here you will find all sorts of tutorials of many aspects of mapping. Most of these tutorials are for either Gimp or Photoshop, and many people have posted ways of doing something close to what is shown in the tut for the other prog.
I would suggest that you check out that Forum, as well as the WIP (Work In progress) Forum to see if any of those have your answer. You might even find other info/tricks that you might like to try out . You might get lucky and someone might actually have tried to do exacty what you are doing.
And welcome to the Guild