Theorically the only thing that you have to do is choose it ase featured Image:
On the post editor, at the right column click on Set Featured Image, then select your image (from your pc or Media library). After that at the bottom of the frame choose: “Use as featured image” this could be tricky since you always use Insert into Post buton, but don’t use it this time, press “Use as featured image” (next to the Insert into Post button), then Save all changes and Update or Publish your post.