First, make sure all hidden files are visible on your system. Then, open the folder for which you wish to add a background image. Within the folder, right-click and select Properties » Customize tab » Customize. There, choose any icon, click Apply and OK.
That process should have created a "desktop.ini file. Open that file with a text editor and add these lines of code:
To customize this according to your needs, edit the path in the last line to reflect to the location of the image you wish to use as the background image for that folder. Refresh the folder and the new background should appear.
Finally, to remove the default icon chosen during the creation of the desktop.ini and restore the default folder icon, delete these lines of code from within the desktop.ini file: