There are built in default behaviors that are programmed into the theme of the site to make creating/editing content as easy as possible. There are times where this default behavior is not preferred and additional steps need to be taken to override the default behavior.
One of these default behaviors is to display the first image uploaded to post as a thumbnail on the “Post Page” context.
To hide the first image in the “Post Page” context, follow these steps:
Find the post and click “Edit this post”
Go to “Display Settings”
Expand the “Post Page styles” accordion
Inside “Custom styles & behaviors” add the following text → image-1-hide
Save the post and navigate back to the “Post Page” context