To make an image appear on this grid you need to add a ‘Featured Image’ to the post (bottom right of the Post admin screen). You can choose to always show these images at the top of your blog posts or not, in the main Divi prefs
This text is the post ‘Excerpt’, on the Post admin page. If you don’t add an excerpt, it will take the first x characters from the top of the post. (The excerpt doesn’t appear on the post, it’s just a summary)
To add a new blog post, got to Posts > Add New. You can use the simple Wordpress editor or you can use the Divi builder if you want to use any of Divi’s modules.