Hi @cra1gyb,
Sorry for the late reply.
Regarding the facebook image, I can explain how it works.
The image is automatically picked by Facebook from your website. Facebook parses your website and looks for certain tags called “open graph” metadata.
Whatever the image URL is, that will be picked by Facebook.
As you can see WP Socializer does not send this image to Facebook and the plugin has no control. There are some plugins which allows you to change this image URL.
If you are using Yoast SEO plugin, then I guess it provides some option to set a different image as an open graph image.
More details on this here – https://simplified.co/blog/design-hacks/open-graph-image-everything-you-need-to-know/
Regarding bulk update for multiple posts, you might have to get some professional help to write some code for toast SEO to programmatically resize the images. Hope this answer was useful in some way.
Thanks,
Aakash