Forcing Default Image for Facebook Posts

I want to force only one image for each blog entry I post to Facebook.

I was able to set an image for "Ultimate Facebook" plug-in so when I autopost to Facebook, the correct image appears. HOWEVER, when I type in a link directly to Facebook, it does not work. It either comes up with the wrong image or gives me multiple options. I have added the following line to my header.php file:

<link rel="image_src" href="IMAGE LINK HERE.jpg" />

I found that guidance here: http://www.gravitationalfx.com/how-to-make-facebook-use-a-custom-thumbnail-image-for-your-website/

Is there something else I can be doing?