Download the GIMP from
www.gimp.org. If you're using Windows follow the links to the Windows installers. Download and install gtk first and then the gimp (as per instructions on the website).
The GIMP provides very much the same functionality as PhotoShop. It's free, open source. I find it easier to use than Photoshop but that's probably because I am used to it.
(In the unlikely event that you're using Linux, you'll probably find that the GIMP is already installed on your system.

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PS. I know that Bird uses the GIMP. That should be as strong an endorsement as image processing software can get.