Exclude advertisements from social sharing thumbnail options

Awhile back I noticed that social linking sites such as Reddit, StumbleUpon and Facebook would often grab advertisements running on my site as the thumbnail image for shared pages. I didn't want that to happen, obviously, so I added a default image link to the header: <link rel="image_src" href="http://gapersblock.com/gfx/default_thumb.jpg"/>

Problem half-solved. Now the default image is the only image sharing sites see, whether there are o开发者_如何学Gother images on the page or not. I'd like readers to be able to choose other images from the page if they want. Is there some code I can wrap our ad invocation code in to "hide" it from sharing sites, or at least make it less? Is there a way to tell these sites' thumbnail generators "Pick whatever you want, except this ad right here, or use this default image if that's all you can find"?

Relevant background: I'm using Movable Type, so Wordpress advice is useless to me. I'm comfortable with HTML and CSS, but a novice on javascript or php.

You would just need to add multiple image_src tags for all the images you want users to be able to pick from.