Hi,
That's really a nice addition. We will consider it for sure. Until then you can just the following snippet in the article layout override to achieve what you want:
if (isset($this->item->jcfields[1]))
{
$document = JFactory::getDocument();
$document->setMetadata('og:image', substr(JUri::root(false), 0, -1).$this->item->jcfields[1]->image->src, 'property');
}
Note that this code is for Joomla articles and assumes that you are using Econa as a custom field and the field ID is 1.
If you are not using Econa as a custom field then you can install Route 66 which allows you to set the article's intro/full image as Open Graph tags.
Regards