Not pulling the correct image into "og:image"

Issue: Create a post with no feature image. Attempt to share on Facebook (hoping Yoast will pull the first image in the blog to use as the “og:image”), yet it pulls an image much farther down the page (possibly because of the dimension ratio?).

URL where you can see this issue: http://www.honeytrek.com/harbour-island-bahamas/ (i want the pink house, i.e. the 1st image on the page, to be the image seen on all Facebook shares.)

https://developers.facebook.com/tools/debug/og/object/ drop that URL in there, and you will see it NOT pulling the first image.

How can I force Yoast to always use the first image in a post, as the og:image?

Thanks,
Mike

https://wordpress.org/plugins/wordpress-seo/