Not only Folders, you can also add cool images to your C,D or E Drives also. Just give it a try and I am sure you will like it.
Here is a step-by-step tutorial :
Step 2) Now open Notepad, copy and paste the following code in it.
[ExtShellFolderViews]
{BE098140-A513-11D0-A3A4-00C04FD706EC}={BE098140-A513-11D0-A3A4-00C04FD706EC}
[{BE098140-A513-11D0-A3A4-00C04FD706EC}]
IconArea_Image=path of the image
Step 3) Now in the IconArea_Image just enter the path of the image that you want to set as background. If the image is in the same folder then no need to give the path simply enter the name of the image along with the extension i.e
IconArea_Image=background.jpg
Step 4) Now save the notepad file and name is as Desktop.INI
That's it ! Just open you folder and now you can see the folder with the selected image as background.
This is how it looks :

To remove the Background Image simple change the image path to "no image " without quotes i.e.,
IconArea_Image=no image