Change the thumbnail image in Facebook post

I would like this photo to appear when posting our form https://form.jotform.us/ronbrownleaders/summit17 to our Facebook Group Page: 

https://www.facebook.com/groups/RonBrownSummit/.  How can I make embed and size the photo into the form so that it will appear as an option for the thumbnail when I post the link?  Right now I don't have the option to use this image when I post the link in a FB post.

Please be noted that the header image will be displaying on the Facebook page automatically when you share a link. However, you may select another image from the form by using the image scroll. I notice that the required image is hidden on the form and so it is not displaying. As a workaround, you may display the image on the form temporarily so that you can select the image on the facebook. Once you are done posting it on the facebook, you may hide the image on the form again.

Hope this information helps!