Image sizing seems to be a problem. From what I can tell from the code, there is just one way to designate the image for a post: by using the meta-data fields in the page where you write the post.
The image may, or may not, be actually used within the post itself. That doesn’t matter.
Sizing the image at 400 x 300 pixels seems to work for all such images except the larger one in the bottom box of the main page (which no longer exists since I’ve modified the theme to exclude it).
This means that you should designate a separate post, with a separate image, for each of those locations.