gimp-savvy.com/BOOK/index.html?node45.html

Grokking the GIMP is a leading book for learning advanced digital image editing techniques using the GNU Image Manipulation Program, the GIMP. The book provides everything needed to learn the GIMP, including tutorials, beginning and advanced techniques ...

photography.tutsplus.com/articles/creating-real-layer-masks-...

Photoshop is not the only program able to do layer masks, a  fast and clean way to mix elements from different images in one final picture. You can do it for free with software like GIMP. Or Perfect Layers 2. In this tutorial we show you how.

blog.irisquest.net/2008/06/gimp-copy-layer-mask-to-another-l...

Did you ever need to copy a layer mask from one layer to another? In Photoshop this is a very easy process, which involves only the mouse and the Ctrl key, but in GIMP things aren't that straightforward (but not much complicated either).

www.youtube.com/watch?v=y_2R-OQddyM

Like the title says - How to blend two images together using Layer Masks. Simple! Special thanks go to Vincent for these images.

www.youtube.com/watch?v=9ErchKGzsNc

GIMP photo editing; beginner tutorial learn what a mask is, how to add a layer mask, paint it, copy & paste a mask and more.

www.gimp.org/tutorials/Layer_Masks/

Layer masks are a fundamental tool in image manipulations. They allow you to selectively modify the opacity (transparency) of the layer they belong to. This differs from the use of the layer Opacity slider as a mask has the ability to ...

docs.gimp.org/en/gimp-concepts-layer-modes.html

GIMP has twenty-one layer modes. Layer modes are also sometimes called “blending modes”. Selecting a layer mode changes the appearance of the layer or image, based on the layer or layers beneath it. If there is only one layer, the layer mod ...