Default Image for Facebook Posting

I am very new to WordPress. I’m looking for advice on how to set what image will be the default when a link to a posting is published in Facebook.

I have not been able to figure out what the rationale Facebook uses for setting the thumbnail. It’s rarely the lead, and sometimes the 4th or 5th image. Is there a way I can tag an image so that it becomes the default? I’ve tried image as thumbnail but that doesn’t work either.

Any advice would be most appreciated.