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

1. Go to the GIMP download page, scroll about 1/5 the way down and click the “Show other downloads” link.
2. You may have to scroll down again to find the “Windows” section. In the Windows section, click the “Download GIMP 2.X.X” link (2.8.10 as of this writing.
3. When the download in complete, run the installer and Bob's your uncle.

1. Go to http://portableapps.com/apps/graphics_pictures/gimp_portable
2. Click the “Download Now” button.
3. When the download is complete, run the installer, tell it where to put GIMP, and Bob's your uncle.