Gimp
free download
Gimp 2.8. A full-featured graphic and image editing application that is freely available for Windows and Mac machines. Note: if you are installing this application on a Mac pay close attention to the download and installation instructions.
http://www.gimp.org/
Gimp Tutorials
Official Tutorials
http://www.gimp.org/tutorials/
GimpLite Quickies
http://www.gimp.org/tutorials/Lite_Quickies/
Gimp Layers
Gimp Layers Tutorial
http://www.tankedup-imaging.com/gimp/layers.html
ePhotoZine. How to Use Layers and Layer Modes
http://www.ephotozine.com/article/how-to-use-layers-and-layer-modes-10659
SumoPaint
web-based
Sumopaint. A full-featured web-based graphics editing and production application.
http://www.sumopaint.com/app/
SumoPaint Tutorials
SumoPaint Support Center Tutorials
http://support.sumoing.com/customer/portal/articles/885742-sumopaint-tutorials
Heather Weaver. Create Digital Masterpieces With Sumo Paint. AppStorm.
http://web.appstorm.net/reviews/graphic-design/create-masterpieces-with-sumo-paint/