Thumbnail images not loading on Stumbleupon

Hi

I have been searching this topic for months now, and can find nothing. For some reason when I (or anyone else for that matter apparently) submits one of my posts to Stumbleupon and Facebook, the featured image (or any image) will not load (as you can see here). I have tried everything, including deactivating every single plugin. I don’t think it is the theme, as I tried an experiment with one of the theme demo posts and it loaded fine.

I tried again today, and as you can see, just a blank icon. It’s not super important, but they same thing happens on Facebook too.

https://www.stumbleupon.com/stumbler/Illusio-5/likes

Thanks for your time.
https://www.lorriewhittington.co.uk/blog/