There are a couple of programs that change the appearance of icons and folders for you.
The first is Colored Folder Creator Pro. Instead of replacing the folder or icon with a picture, it colors your folders in a variety of ways or puts pictures on the actual folders while retaining the folder shape. This is handy if you still want the folders to look like folders, or if you are too lazy to follow the steps and do it manually. However, Colored Folder Creator Pro does cost $5 to use its full version. Personally, I'd rather just do it myself.
Download Colored Folder Creator Pro.