I'm having some issues with what picture is used, when posting to Facebook. Initially I had set a default og:image, so then I omitted this. However, now when I try to post something to Facebook (from Facebook itself) it's still the old og:image being used.
Example using Yoast SEO:
(shows a Porsche 911 from the front page, the original og:image, even though og:image has been omitted from the html output)
Not using Yoast SEO:
(shows the proper slideshow images and lets me choose)
Can you help? There's currently no WordPress caching plugin enabled, however the ISP uses Varnish.