Posting blog posts to Facebook help

It's quite a basic one I expect this one.

When you try to post a blog post link to Facebook etc using something like the WP 'Add To Any' plugin, Facebook looks at your site in order to grab an image from it to display along side the post.

Where can I add the Meta Data so that it displays my site's logo and not a random advert or something from the same page of the blog? I expect it goes in the Head Meta Tags, but I've no idea where these are or where I put the code.
Sorry for being a bit simple!
Ross :slight_smile: