It is possible to set a background picture to individual folders.
The folder must contain a desktop.ini file and it must have the "system" attribute.
You can create a desktop.ini in notepad and use the attrib command to apply the required system attribute.
The easiest way to do both is to rightclick the folder, choose customise and change the icon. You can change the icon back if you want later, but you now have a desktop.ini file to edit.