Facebook share url thumbnail problem

Possible Duplicate:
How does Facebook Sharer select Images?

Want to have share thumbnail when user shares site url using share button on site which uses http://www.facebook.com/sharer.php to achieve that.

Site dose not have img tags, all images are defined in css.

I added meta tags for image:

<meta property="og:image" content="/images/branding/branding-logo.png"/>

and

<link rel="image_src" href="/images/branding/branding-logo.png"/>

but it's not working :(