tag tells Facebook which image to use on your card.
It requires the URL to an image, so it may be best to use one of the image modules to upload an image first so that you have a good one to provide for facebook.
So upload an image somewhere and copy that images url for use in the Facebook snipit.
An easy way:
* open the blog editor
* add an image to the blog post via the editor image upload module
* set the image to something quite large, maybe 800px
* open the editor source code tab that looks like < >
* copy the image url:
We need that URL to use in the meta tag module.
You can close the blog post now, don't need to save it, the image is uploaded.