I actually found your post by way of another of your posts and have a question after I answer your question that I hope you can answer.
To answer your question
For best results the Facebook image size should be 600 x 315 at the current time.
It can fluctuate apparently, but at least use 200x200 which is what is was previously. I have a photoshop source that I pop shared images into and use the 600 x 315 specifically for every post with clients.
You can check Facebook open graph page for these details here.
You can check Stack Overflow here for more details and some conversation.
Did you find an answer to this post you left here?
I'm searching and having no luck.