Video thumbnail image not saving?

I’m sure this is easily solved. I’m trying to add a thumbnail image to an embedded video on my landing page but for whatever reason, it won’t upload to my page after saving and publishing it? I’ve tried JPEG and PNG versions but it still won’t save but the embedded video is fine.

A message in the image fields within the Media Library says that the image fields marked with an asterisk must be completed but there aren’t any fields with an asterisk so that shouldn’t be a problem so I can’t think what else it can be?

Can anyone help with this please.

Many thanks.