Check out this result of debugging an article using Facebook debugger.
It comes up with this warning:
og:image should be larger. Provided og:image is not big enough. Please use an image that's at least 200x200 and preferably 1500x1500.
But if you open the image displayed in the "og:image"-field below, you can clearly see that the image is big enough - 700x350 px.
This results in Facebook picking random images when I'm publishing articles on my wall.
Any idea why this happens?
Edit: Could the problem be caused by my CMS redirecting the url of the image?
Try to add this:
<meta property="og:image:width" content="200" />
<meta property="og:image:height" content="200" />