As explained previously, the other method of changing your icons, which takes rather longer to do is, to change the icons one at a time. I actually prefer this method, as it enables me to choose really good icons as I go about my daily Moodle tasks. This can involve choosing icons from more than one Moodle icon set, or simply going and finding a great set of icons that you are allowed to use on the Internet.
With this method, you have to make sure that you are consistent in choosing icons, meaning that you should try to make sure that they look similar. You can find many icon websites on the Internet, but to get you going here are a few examples:
http://www.maxpower.ca/: Click on the free icon link in the menu bar
http://www.kde-look.org/: For Linux desktops, but some good ones nevertheless
http://www.rad-e8.com/: Stylized and minimalistic
http://iconfactory.com/freeware/: Great unique icons