This post is for people who have a Facebook brand page, not personal Facebook profile.
When you post a link from your website or blog to your Facebook Wall it pull sthrough a thumbnail from that website. You’ll find that it is often an unconnected image or is badly sized. You can prevent this by including a custom Facebook image on your site or in your post. here is an image at the size that you need. The dimensions you need are H:68 x W:90 pixels.