How To Select a Featured Image in WordPress
When you’re editing a post or page, scroll down and locate the “Featured Image” section. It’s usually located in the right column near the bottom. If it’s not visible to you, scroll to the top of the page and click “Screen Options”. This will expand the menu. Make sure “Featured Image” is checked. This should ensure it is displayed below.
To set a featured image, just click on the “set featured image” link. You’ll be brought to a screen where you can select an already-uploaded image from your media library, or you can upload a new one.
For best results, make sure your image is a decent size. I would suggest making sure the image is at least 600px on either side. If your featured image is too small, it will not fill the space properly in areas that call for your featured image.