[BirdOPrey5]

Honestly I never used it with a fanpage so I don't know what may or may not be possible with one.

Another option is to add the following lines to your headinclde template:

<meta property="og:site_name" content="{vb:raw vboptions.bbtitle}"/>  
<meta property="og:image" content="http://www.yourdomain.com/images/facebook_image.png"/>
<meta property="og:type" content="article" />

You would replace the URL to the image with a URL to an image file you created that you want to be used. Something roughly square or rectangular, between 100x100 and 300x300 pixels, facebook will auto resize it.

Also once you do this give it some time, facebook caches your image so it may take a while for this to show up/change.

But again I've never tried it off a fan page.