publicize taking wrong image

As per others here, I’m getting the wrong image uploaded to Facebook with the publicize option.

It seems to be taking a picture at random from a side widget which displays online users.

The site is https://encast.it

I have a site icon set up but this *sometimes* displays but often does not in place of random user images.

Any help appreciated!

https://www.ads-software.com/plugins/jetpack/