http://www.imagefap.com/mydesign.php Click the drop-down box next to View other block and select Background. Then copy & paste the URL of the image in the Background image box, choose your Repeat & Position settings then click Save Changes.
Most likely the wrong URL for the image. You need to right-click it and select View image... then copy & paste that URL (starting with fap.to or x.fap.to, NOT imagefap.com).