PaintTool SAI

Originally a Japanese program, it has been ported to support English. A sturdy arsenal of many different features and tools allow you to edit photos with ease with this simple program

Features:

• More than one document can be edited simultaneously
• Zoom and rotate the interface
• Available in both Japanese and English languages
• Mirror your drawing without mirroring the image
• Marker, watercolor, airbrush, pen drawing tools
• Easily customize and store drawing presets
• Options dialog allows you to fine-tune the way that you perceive to be best to use the program, such as custom brushes
• Do work on multiple different layers, which can then be grouped and can have masks of opacity applied to them. Also, you can clip layers to a lower layer in order to mask them.
• You can customize the selection tool, similarly to a drawing brush. You will also find more traditional selection tools such as simple square, lasso, and magic wand. 
• The program contains a robust set of transformation tools, such as rotate, resize, move, and free transform. This can be done on multiple sections of the image at once. 
• Files can be exported in many different formats, such as the program's original format, .SAI, as well as .PSD and .BMP

Simple to use and not very resource intensive

If you're just getting started editing photos, the simplicity of this program may be a good fit for you. Or, if you are a seasoned editor, this program can also be a good fit because it will scale down or up to your skill level. The interface is customizable, and it boasts an impressive amount of features. No matter what your desired usage is, you can be assured that you will be able to find something in this program that can satisfy you.