Ah, the Exhibit theme bumps the images up next to the container.
To counter this, add this to a custom CSS plugin:
.separate-containers .post-image, .separate-containers .inside-article .page-header-image-single, .separate-containers .inside-article .page-header-image, .separate-containers .inside-article .page-header-content-single, .no-sidebar .inside-article .page-header-image-single, .no-sidebar .inside-article .page-header-image, article .inside-article .page-header-post-image {
margin: 2em 2em 2em 0;
}