LiteIcon is an extremely simple utility for quickly replacing icons in Mac OS. Use the app to replace the standard system icons with your custom ones. All you need is to "grab" the new icon with the mouse pointer and drag it to the folder /program, after which it will agree to make changes. LiteIcon will do the rest for you!
Before using, be sure to download the version for your version of the operating system. If you are using a fresh distribution, try IconChamp.