Enhance>Adjust Lighting>Levels
The most important skill in digital art is layering. In GIMP, you layer by following these basic steps:
Close all windows except the GIMP tool box and active image window.
Open the Layers Dialog box from the active image window: Windows>>Dockable Dialogs>>Layers.
When you select a layer in the Layers Dialog box that layer comes to the top of the image window.
Clicking the Up Arrow in the Layers Dialog box changes the layer number of the selected layer.
Roger S. King 