Here is a guide to using GIMP 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.

In what follows, I am assuming you are using PNG images for all your image resources. With small changes, everything should apply to other formats as well.

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 down past the “GIMP for Unix-like systems” until you see the “Show other downloads” link. Click that link.
2. After clicking the link, scroll down again to find the “GIMP for Windows” section. In the “GIMP for 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.

1. Go to the GIMP download page, scroll down past the “GIMP for Unix-like systems” until you see the “Show other downloads” link. Click that link.
2. After clicking the link, scroll down again to find the “GIMP for Mac OS X” section. In the “GIMP for Mac OS X” section, follow the instructions in the “Native builds” section.

1. Go to the GIMP download page.
2. Follow the instruction under “GIMP for Unix-like systems” to install GIMP for your distribution.

1. Open the image you want to convert in GIMP.
2. Go to File > Export … and in the resulting dialog:
   1. Be sure Select File Type is set to (By Extension).
   2. Enter myImage.png (Make sure the extension is .png)
3. To save changes you have made to a PNG file, go to File > Export to myImage.png

Often times an image will have extra borders or padding around it that you won't want. (It will make sprites larger than they need to be–which will affect collisions and other things.