Tumblr Link Theme Operator and Thumbnail
Tumblr recently added some new theme operators to allow for link thumbnails.
I presume this works the same as video thumbnails work and may only be available if the linked site creates the thumbnail.
{block:Link}
{block:Thumbnail}
<img src="{Thumbnail}" alt="{Name}">
{/block:Thumbnail}
{/block:Link}
Reference: http://www.tumblr.com/docs/en/custom_themes#link-posts