You have to right click the folder, then click "properties".
After that, you go to the "Customize" tab and under folder icons, click "Change Icon".

You'll then have to browse to the directory that you have the images, and choose one. The images also have to be in .ico format, so that they can be used as icons. I downloaded a 75mb pack from a site that has like 3,000 icons or so.
If you have any questions let me know.