Dave, when we click on the option to make an image the main thumbnail, it says" This action will over-rule the thumbs-up votes and can only be reversed by you or another moderator of this plant." Well, there was one I made a main thumbnail and now wanted to change/reverse it but I do not see an option to do that.

I think all you have to do, Sue, is choose another image to be the main thumbnail. That will override the earlier choice.

Zuzu is right, you have to choose a different one.

If there isn't a different one to choose, you'll have to let me know which image it is and I will manually remove the override.

Ahh, well of course, I didn't even think of that, it makes sense!