thanks for getting back to me and big thank you for the screenshots! I have installed some of those plugins that could affect our plugin but unfortunately I have been unsuccessful in replicating the issue. If I am not mistaken you are using the "Outreach" theme which is not free to use, so I can't really test this completely on my side. However you could try solving the problem with custom fields:
1. first, go to your create-new-post page or —even better— in one of your previous already-published-posts. On the right top corner you should see the 'Screen Options' button; click on it for the drop-down menu to open. After that, tick the box next to 'Custom Fields'.
2. now you should be able to see a new box below the editor called 'Custom Fields'. Create new custom field, name your image that you would later like to use it as a thumbnail for this specific post (something like "My thumbnail"), enter the URL of the image you would like to use and finish with the click on the button "Add Custom Field". Of course, don't forget to save this draft, or publish the post when you're ready.
3. after that, visit the settings of (y)our plugin and scroll down to the bottom. The option called "Use custom field for thumbnails" should now be available, tick the box next to it, and select the value you've set up before (if you used "My thumbnail", select that one).
Let me know how it goes, I am eager to hear the results!
Take care and have a nice weekend,