Featured Image and Thumbnails

OK, this topic may have already been addressed… but if so I couldn’t find the answer.

Here is my dilemma:

I am trying to use one image as both the thumbnail for the posts feed page as well as the image in the actual post. I have tried several different ways, but either end of with 2 images or only a thumbnail and no image inside the post itself.

Here is a link to my blog page to help illustrate what I am trying to say:
https://www.vaughnindustries.net/blog/

Can anyone help me with this? I am trying to use the featured image to post my photo. Is there a plug-in I need to use to display the image in this way?

Thanks for any input!