Paint.net is a very good painting and photo-editing software. It supports the .bmp, .jpg, .gif, .png, .tiff, .tga, .dds and .pdn file formats. Paint.net kan draw in layers, has an infinite number of Undo's (limited only by your amount of memory and harddisk space) and many proffesional effects that can be used to improve and alter your digital pictures. You have functions that removes the red eye effect from your pictures, and other functions that can make your pictures look like an oil painting or a sketched image. With Paint.net you can work in layers. That means that you draw on layers that lay on top of each other, and you can adjust the alpha level, or transparency, of each of those layers to get to the final picture you want.
Paint.net was originally intended to be a replacement for the Paint-program that is shipped with Windows, but was abandoned for that purpose. Since then it has continued to evolve as a Open Source project, which in short means anyone can download the source code and make improvements that all other users may benefit from. You can download the source code from paint.net's home page if this is something you would be interested in.
To install Paint.net you'll need: