• Drop Shadows
• Unsharp Mask
• Adjusting Contrast
• Bump Mapping
• Image Watermark
• Gradients Tutorial
• Colour Enhancing
• What are Layers?
• GIMP Interface

GIMP Contrast mask tutorial

Desaturate and invert the overlay layer

On the image toolbar desaturate the layer

Layer » Colours » Desaturate(GIMP version 2.2)
Colours » Desaturate(GIMP version 2.4)

Then on the same toolbar invert the layer

Layer » Colours » Invert(GIMP version 2.2)
Colours » Invert(GIMP version 2.4)

You should notice the image is now a lot clearer.