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.
Gimp Layers Tutorial
ePhotoZine. How to Use Layers and Layer Modes
Sumopaint. A full-featured web-based graphics editing and production application.
SumoPaint Support Center Tutorials
Heather Weaver. Create Digital Masterpieces With Sumo Paint. AppStorm.