Warning: tutorial may contain a whole ton wee bit of Lost Kate/Sawyer-ness and spoilers for Season 3. You've been warned.
to
in three easy steps! (Curves)
1. Take your base in GIMP.
2. Duplicate the base layer and set it to 'screen'.
3. Take out the curves tool (Layer - Color- Curves) make sure you are working with the screen layer. Set the Value channel to x: 64 and y: 137
You're done! I added some text. If you don't know how to use curves, I'd take a look in the archives at
icon_tutorial. I'm sorry if I didn't explain this well enough, please ask questions if you have any.