Error sharing post links on Facebook

Over the last 2 weeks when I shared a link on Facebook or Digg the thumbnail from the post didn’t appear, In fact it was the image I have on my Facebook fan page and my twitter page but I don’t have that picture on my webpage so I don’t know where it came from.

I have tried adding plugins that said they can add the code to share thumbnails on Facebook but they didn’t work.

Now I have uninstalled those plugins and I have made it worse as if I share a link on Facebook it just says the words ‘title’ and ‘description’ along with the url of the post and still no thumbnail.

Can anyone tell me what code I need to add to my theme templates so that the title, description and images appear when I share a post?

Thanks