PopUp showing Blank

I've gone through these support tickets and haven't found anyone with this issue (outside of one other person who just didn't reply).

My PopUp is showing up, and the Active Rules are working properly as well.
The only issue is that the content I attempted to add doesn't show up. It simply shows up as a white box.
Disabling JS and selecting 100x100 as its location works as well.
So what am I doing wrong?

If I could get the content to show up I'd be all set!

Thanks,
MENet

EDIT:
Here is the code I'm trying to display.
<a href="http://www.mymiddle-earth.net/register/"><img src="http://www.mymiddle-earth.net/wp-content/uploads/2012/02/join-now_500_350.jpg"></a>