Lately I wrote on my general blog a couple of photo editing tutorials, both of them are targeted at GIMP, since this is what I use on my day to day work. Of course, the principles are general and can be applied to other similar tools.
The first one is about watermark removal on photos submitted to Wikipedia, this is something perfectly legal, since those pictures are submitter under a Creative Commons Attribution Share Alike license.
The second tutorials refers to creating customs grids to be used on photo editing but also in graphic and layout design.
This work is licensed under a Creative Commons Attribution-Share Alike 3.0 License.