Help changing an HTML code in Tumblr

I have a new tumblr blog.

When I copy and paste the link to facebook of a specific blog post, a generic image comes up and does not allow me to change the image with the image arrows on facebook.

I have researched and read that the 'meta' code for my blogs HTML needs to be changed so that Facebook, or any other medium, selects the same image of my choice every time.

I would like the code updated so that when the link is copied and pasted, facebook selects the correct image. I have googled this and it seems fairly easy but I do not know the terminology nor am I skilled in code...

I have uploaded an image of what is coming up now, and then the image that it should be.

Here is a couple links with an example of the information I have found while researching:

https://www.facebook.com/help/community/question/?id=10200814000150949

http://www.freebookings.com/blog/social-media/control-the-image-and-text-shown-in-a-facebook-status-update/

http://stackoverflow.com/questions/19885511/show-particular-thumbnail-on-facebook-when-sharing-tumblr-link

Thank you