The featured image appears as the thumbnail of your post on social media, your website homepage and all post lists (like latest post, for instance).

It is also a since Google's documentation states that every page must contain at least one image.

On the homepage, category pages, and sidebar, the featured image that is associated with each article will be cropped at a 4:3 aspect ratio.
 
Add the feature image on the post editor right sidebar, on the highlighted spot below. 
 
🟠 Please not that if you want the featured media on the top of your post, you need to add it manually.

Size

The image uploaded has to be a minimum size 1600px (width) by 1200px (height). Make sure your image resolution is 72ppi (pixels per inch) or higher. This will guarantee a good image quality and fulfill Google News standards. Please do not upload images larger than 2560px in height or width in order to optimize hosting disk space.
 

Orientation

We recommend a landscape orientation as close as possible to the 4:3 aspect ratio, so your image will display optimally as an article thumbnail and as a pinned article image. If you upload an image with a vertical orientation, it will be cropped equally on the top and bottom to accommodate the 4:3 ratio, and may result in an awkward cropping.
 
If the width of the image is higher than this recommended aspect ratio, the left and right edges of the images will be automatically cropped, hiding some potentially important details.
 
 

Format

The recommended format for photos on the website is .JPEG or .JPG. Images in this format can be effectively compressed, and the file size will be smaller than a .PNG.