If you are using the additional images instead of the legacy thumbnail/full size image because you have implemented the image machine, you could use the Emporium Plus Tool Kit to grab that image url and submit it as the Facebook image, defined in the head section of your page. If there is not the additional image, you could fall back and use the legacy full sized image.
Code:
<mvt:item name="toolkit" param="customimage|big|l.all_settings:product:id|default" />
<mvt:if expr="g.big">
<meta property="og:image" content="http://www.YOURDOMAIN.com/mm5/&mvta:global:big;" />
<mvt:else>
<mvt:if expr="l.settings:product:image">
<meta property="og:image" content="http://www.YOURDOMAIN.com/mm5/&mvta:product:image;" />
</mvt:if>
</mvt:if>
See http://www.emporiumplus.com/1AA00223.html for more explanation at #134 on how to use that function.