The linked background may not load from following reasons:
1. the background picture is not available on the specified URI
2. the background picture is available but the format is not compatible with maps
3. the web server cannot connect to the specified URI
I suggest following these steps to find the cause of your issue:
1. check whether the URI is available as image in your browser and it is GIF (not animated), JPEG, PNG, or BMP
2. check whether the picture is available on the server
3. if it is not, check connection, firewall, and whether a proxy is used.
If none of these helps, please contact our support.
Thanks Michael, I'll give it a shot and let you know how it goes.