At the top of this page is a featured image. It’s the same images you see on the homepage grid. If you don’t have a featured image then you will see a placeholder on the homepage and nothing at the top of the applicable page.
To add a featured image use the ‘Featured Image’ function in the right hand column of the post/page editor:
When your posts display on the homepage the featured image will have an overlay that shows the post title. You can add a second line to the overlay by adding Alt Text to the image. To do this go to the media library and select the image you want to edit. You will now see options on the right to add Alt Text:
You can add a Title, Caption and Description if you want but these aren’t used for the featured image.