Hello,
Thank you for contacting Rank Math and bringing your concern to our attention. I’m sorry for the delay and for any inconvenience this issue may have caused you.
Unfortunately, the preview of the image thumbnail will only show in the social preview snippet if you have set a featured image for your posts/pages:

Social Preview Snippet Editor can only display the featured image that you have set, but please note that it is just the preview we are talking about. Both plugins add the required Schema needed to show the image in actual SERPs.
You can verify this on Google’s Rich Results Test tool.
I hope that helps. Thank you, and please don’t hesitate to contact us anytime if you need further assistance.
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.