Limiting the number of thumbnails on facebook share

So I want users to be able to share this link on facebook but I want only a specific picture to be the thumbnail for this sharing picture and no other images that appear on my web site

I read the documentation about facebook share and I got this

http://developers.facebook.com/docs/share

Basically what I understand is that

<link rel="image_src" href="thumbnail_image" / >

should do what I want. However, I cannot get开发者_StackOverflow this to work. My site is programmed using rails and, inside the layout, I have the above tag to take in an image url coming from a model instance.

Does anyone know how to use this properly? Am I misunderstanding something about this tag? I want to disable user from selecting through the images that I have on my page and just allow the user to share with only 1 image which I have to specify.

To specify a specific picture you will need to add open graph meta tags to your page. Take a look at http://developers.facebook.com/docs/opengraph/.

The open graph tag for your case is: og:image