Facebook doesn't allow any 3rd party app to specify which image is used with a Link type post, where there's a link attached.
However, they do allow you to force an Image post type and specify the image. It's a change they made late last year.
What you can do is upload your own image and attach it to your post, then switch on the 'Attach media' switch above the Facebook preview. It will move your attached link to the end of your message, so it still links.