GIMP 2, available at GIMP.ORG, is a free Image Manipulation Program that has many of the capabilities of Adobe Photoshop and other High-End, expensive image manipulation programs, but won't set you back an arm and a leg (or, with Photoshops new "subscription" model, an arm and a leg and an arm and a leg and...) Not only is it highly capable out of the box, it is also fully "extensible" if you are technically savvy and want to try your hand at writing custom image manipulation code.
No, It's Not a Free Photoshop
Having used both Photoshop and Gimp, I can flatly state that GIMP is NOT a "free Photoshop" knock-off. The user interface is very different, and takes some getting used to. If you are familiar with Photoshop, the controls may be a little confusing at first. The primary stumbling block that I found is the difference between the Load and Save functions.
Photoshop, and most other Image Software I have worked with, lump "Save" and "Save As" together with image file export. To save an image that you are working with as a BitMap, or a JPEG, or a GIF, you would simply "Save As." GIMP has a special "Export" menu selection for saving to "non-GIMP" formats.
While you can "drag" images into Gimp as you would most other programs, the "save" options are "Save..." (to save to an ".XCF" file) or "Save As..." (to a different XCF File). You can chose to "overwrite" the file you dragged in (which, on most other systems, is simply "save"), or you can "Export" the file.
It's a little confusing at first, but once you get used to it, you'll understand the logic behind it all.
Yes, it is a Powerful Image Editor
That it is not Photoshop is perfectly apparent. That it has many of the same features as Photoshop is also apparent. You can adjust color levels, add layers, clip, copy, paste, crop, and resize your images. These are essential capabilities for creating textures to put on your 3D models you are creating in Anim8or, or adding to the UDK (Unreal Developer's Kit) World that you're working on.
You can also convert files from one format to another using the "Import" and "Export" features. For example, most web sites use the JPEG (.JPG) standard for images, whereas many Game Tools use the TARGA (.TGA) standard. This mostly has to do with differences in compression and image storage standards. Suffice it to say that there's some extra information in TARGA files that you can use in your game content that would not be there in a standard JPEG file.
GIMP Is Extensible
While this may not be everyone's bailiwick, GIMP has the advantage of being scriptable and extensible, so in the event you find a feature that doesn't behave the way you want it too, you can modify that behavior by writing your own code and incorporating it as a plug-in or add-on.
Help Is Always Available
GIMP also provides users with Help Manuals, Tutorials, and a robust user community who will help you if you run into problems with your work. I recommend starting with the Tutorials, as the manuals are a bit daunting, and the user community, while friendly, will often refer you to the tutorials anyway for the most basic of inquiries.
The next article will present some information on the Unreal Design Kit referenced above, which is where you can use some of the output from GIMP and Anim8or to build Game Content!