Just been asked how to force facebook to use a specific image when posting.
Facebook appears to select an image at random (I'm not sure what the algorithm is).
Quite often this isn't the best image to use or even completely wrong like Twitter Tweet now logo.
Firstly, you can check which image facebook will use for an given url at:
To force facebook to use a specific Image you need to reference it in the head section.
<link rel="image_src" href="http://www.markthompson.me.uk/Custom/Themes/Standard/images/markthompson_thumbnail.jpg">