My plugin also does it for you.
https://xenforo.com/community/resources/facebook%E2%80%99s-open-graph-image-or-og-image-for-forums-and-threads.5503/
It is for XF1.5 but can extend this if there are people looking for something similar.
I had developed a plugin to import Forums using a CSV file. If it works for you, can share it with you but if you want me to be doing it for JSON, shouldn't be an issue either.
You don't need an ad manager in XF2 because the functionality is built in out of the box. Here is visual ad guide for XF2 https://xfpoint.com/guides/xenforo-2-ads-visual-guide/
Changing the passwords won't be enough and it may be that they have some malicious files in your server. So a restore and other stuff may not help and you have to close all the backdoors as well. If you want me to look into it, will be more than willing to do it.
The screenshot is for threads. For forums, you have to upload the images you want to set as OG Image using FTP.
For threads, you can edit the thread and put an image for the thread.
My plugin does what you are looking for but you need to assign an image to forums and threads will default to forum og image instead of avatar. Avatars aren't very social media friendly. Check...