For a Canvas or Freeform email, you can set the default image for Facebook social sharing by putting the below code in the head section of the HTML code (after <head> and before the head section closes i.e. </head>). Keep in mind, this will only work in a Canvas or Freeform email. You will also want to add a title between the title tags as well in the HTML code which is located at the top.
<title> message </title>
<link href="http://example.com/image.jpg" rel="image_src" />
The “image_src” link rel is one of the key tags that Facebook will look for when associating images with shared links. Of course you need to change the “href” to the URL for your own uploaded thumbnail image. This is located in the Library when you click on an image, the URL will be displayed that you can copy. Or if you are hosting your own image, place your own URL in the code.