Facebook Thumb Choice Problem

Hi all, my blog is La Pêche Fraîche.
The problem I am running into is when I share a post to Facebook, the choices for the thumb are not great. Most of the time, I just want to choose the first image in a post, but for some reason, my options are very limited (in this post, for example, I have 12 images, and I’d like to use the first one, but I’m getting three random image options instead). All of my photos are scaled to 600px by 899 px so I know they can fit into the large Facebook link image. This wasn’t an issue previously, but has recently cropped up. Any help is greatly appreciated!