Most Times, When we feed our blog post to Facebook, The Snippet been displayed will be different from that of the post. I actually faced that same issue and i came up with this solution.
First of all you have to backup your template although you may choose not to if you think you can go along without mistake.
After that, Edit your HTML and search for this <b:include data='blog' name='all-head-content'/>
Right Underneath the Above code, copy and paste this there
<b:if cond='data:blog.postImageUrl'>
<meta expr:content='data:blog.postImageUrl' property='og:image'/>
<b:else/>
<meta content='Put URL of alternate image' property='og:image'/>
</b:if>
You are to the alternative URL of the image you want to appear on Facebook just incase you published a post that has no image.
Note that the first image in each post should be 200 x 200px or larger if not that alternative image will be used on your post,
If you think this article has helped or will help please don't forget to share and also if you've got questions, subtractions or additions, please don't hesitate to comment. Thanks
