You can't to put it simply, it you want an image for social sharing on Facebook then you would need to use the special og: meta tags which aren't XHTML valid but they allow you to supply an image for sharing your site on Facebook.

http://davidwalsh.name/facebook-meta-tags

To test any changes you make to the meta values you will need to use the Facebook debug tool which clears the storage cache for your allowing the new information to be re-cached.

https://developers.facebook.com/tools/debug