Old way, no longer works:
<link rel="image_src" href="http://yoururl/yourimage"/>
Reported new way, also does not work:
<meta property="og:image" content="http://yoururl/yourimage"/>
It randomly worked off and on during the first day I implemented it, hasn't worked at all since.
The Facebook linter page, a utility that inspects your page, reports that everything is correct and does display the thumbnail I selected... just that the share.php page itself doesn't seem to be functioning. Has to be a bug over at Facebook, one they apparently don't care to fix as every bug report regarding this issue I've seen in their system all say resolved or fixed.