Check out this result of debugging an article using Facebook debugger.
It comes up with this warning:
og:image should be larger. Provided og:
First of all you need to upload an image that is more than 200 x 200 (for instance I've uploaded the maximum size that is 1500 x 1500). Some people say that the image width and height should be the same size. I've never took chances to check if it's right or not although.
After updating the image size the problem still exists and you'll need to add certain parameter to the og: image link in order to make Facebook debugger update the cache of the image.
For instance:
This is the parameter: ?t=12345?
Wait for about 5 minutes and the issue will be fixed.