Thank you for your interest in Firaxis Games and using our in-game assets. The easiest way to make use of our assets without worrying about legal restrictions and disclaimers is to download the Alpha Centauri fan site kit and pick and choose among the many graphics and images provided. In addition, you are free to download specific images, characters, graphics or sound effects from our website. All assets are owned by Firaxis Games, Inc. and you may not sell, distribute, manipulate, or otherwise alter the image or sound effect. 

You'll need to include the following legal verbiage either next to each use, or on a "credits" page. 

[Image/Sound Effect] from Sid Meier's Alpha Centauri. Sid Meier's Alpha Centauri copyright 1998 Firaxis Games, Inc. All rights reserved. Used by permission.