If you want to use the XML field (folder's description field) then you'll have to create an image object. For example, create a new image object in the media part of the system. Edit the folder and add the image as a related object. Copy and paste the <object id=...> code into the XML field.
Another way would be to override the the folder template for that exact node (and nothing else). In the override template, you would place your own/custom HTML code which could easily bring up an image (using the img tag) from the images folder of the design that you're using:
