Setting Post Image in Facebook

I am trying to have only one image for every post made on Facebook that links back to my site. I added the following line in my theme's header.php: <link rel="image_src" target="_blank" href="http://infowarriorsunited.com/wp-content/uploads/2013/04/iwuOG.jpg" />

That seems to have helped bring it up, but now it gives me either 3 possible options or defaults to one that is wrong. Here is a link as a test case to show what I mean: http://infowarriorsunited.com/explosion-shakes-central-prague-as-many-as-40-injured/