Good evening Mats and welcome to the Events Calendar Support forum!
Thank you for reaching out to us. We are sorry to hear about the missing Facebook image issue you have been dealing with.
We will gladly try to assist you on this topic.
In order to investigate possible errors, I used the Facebook’s DeBugger tool for Developers.
You can try it for yourself if you want.
It turns out that there is an error on your single events pages (and maybe others). The open graph (og) type property appears to be missing.
A lot of people fix this issue through the use of a SEO plugin where you can define the metadata passed to Facebook (e.g.: Yoast SEO).
If that doesn’t work for you, as a first troubleshooting step, could you please provide us with your system information in a private reply ?
Have a great day!
Geoff B.