Here is a temporary trick i have found to overcome this heartbreaking issue :
1- wordpress.com users are unable to change their PHP head sections as suggested all over the forums on www as a solution for this issue
2- facebook picks the related thumbnail itself without asking your decision when you try sharing the link of your awesome blog post
3- changing browsers do not work
4- clear all your cache, delete all cookies, open two tabs, one for facebook, one for wordpress admin panel
5- log in wordpress, delete all the photos in your related post, which means delete them also from your gallery
6- upload the only photo you want it to be your thumbnail while sharing the post on facebook
7- place the photo in the post where it should be, update your post
8- log in facebook, write down your posts link, you will see the only photo you uploaded, attach BUT DO NOT SHARE IT YET
9- upload your other related photos with your post in wordpress, place them in their places, DO NOT UPDATE YET
10- Share your awesome post in facebook
11- Update your post in wordpress with all photos on
Voila. This worked for me.
Bug of this solution : If you try sharing the link once more by copy-paste the link, facebook will again pick whatever it likes as a thumbnail. So try re-sharing the post by just hitting the share button on the right thumbnailed one you shared previously