Hello, the checkered flag to show the first image in the post and this image is from a remote server that is not visible – it is worth to fix it because many people use their own external gallery.
So far you’re the only one who told us about wanting to use images from an external gallery.
The only workaround I see is to set the desired post image as a featured image (which you DO have to include locally in your WordPress).
If you like our creations, help us share by rating them on WordPress.org. Please check the available documentation and search the forums before starting a new topic.