Adding a Thumbnail to your post is how you get a related image to appear on the Blog Page.
1: Navigate to Content and Blog via the side menu.
2: Click on the post you wish to add a Thumbnail to.
3: Above the editor window that appears, click on the ' Settings ' box.
4: Click on the ' Add Image ' button that appears halfway down the page.
From the pop-up, choose your image.
5: Once you're happy with the image choice, press ' Save Changes '.