Due to Facebook's recent policy changes for developers, all content on White Label Applications must be hosted on secure servers (i.e. https instead of http).

Instead of calling a background image like this:

#contest_title {background: url(http://imageurl.png) 0 0 no-repeat;}

Simply remove the http: so that it reads like this:

#contest_title {background: url(//imageurl.png) 0 0 no-repeat;}

Please note, this will only resolve the issue if your hosting server supports both http and https. If your images are not hosted on a secure server, then you can use a free service like Google's Picasa to host your images, as seen here.