Asianfanfics has blocked images from websites other than the main ones used for pictures for example: tumblr, photobucket, tinyimage, etc due to viruses that users have apparently encountered in the past.

If you want an image as your background, save it on your computer, upload it to photobucket/tinyimage/imgur/etc (maybe even tumblr) and use the url you get for that. If you do that your background image should show up :D