Image resize in posts not working

Hello
I don’t think this specific issue has been discussed here recently.

In a post, when I add a picture in the editor, and change the size from thumbnail to medium or large, it adjusts correctly in the editor, but displays a thumbnail size picture on the website.

I know this is specific to Hemingway as I swapped themes to twenty fifteen and the pictures display correctly. I love Hemingway and don’t want to change. WordPress beginner so changing code without guidance is beyond me…

Help?!?
Thanks in advance…