I have played around with the Boardwalk theme on my test site. The following is not specifically mentioned on the theme info page, but is what I have found with this theme.
Boardwalk takes the featured image of a post/page and crops it to the window size of the device the post/page is being viewed on. So, regardless of whether you are viewing the site on a mobile or not, the image will be cropped off to match the window it is being displayed in.
Since you are on a free plan with WordPress.com, my best solution for you would be to switch to a theme that does not crop images like that. Some themes I can suggest would be… Karuna Affinity Lovecraft