Hello Harvey-
There are two steps to making your background image do this. First, we’ll need to modify your blog pages so that they have the right CSS classes for using the background. Navigate to Customize > Advanced > Custom JS & CSS and add the following line to your Custom Theme JS:
jQuery('body.blog').addClass('single single-format-standard').removeClass('blog');
Then, add this line to your Custom Theme CSS:
.single .main { margin-top: 10em; }
You can adjust the value “10em” to fit your needs for the size of the image.