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Here is a guide to using GIMP, an open source image editing program, for doing some of the more common kinds of things you would need to do when editing images for use in games.
GIMP may or may not be the best tool for doing this. But it's open source (something that's important to me), it's available for all major desktop operating systems, I know it, and it works.
You've got two choices if you are going to use GIMP on Windows: you can install it as a regular program or you can use the portable version