Why use a Featured image in WordPress? The Featured Image (also known as post thumbnail) allows a representative image for posts. When you post your blog it will show up in the post at the top, this may depend on your theme design. Use an image that is representational of your post. It is also helpful if you post it to FB, Linked-in Etc as the software will pick up the image to use in the post.
Make sure you are in your post and select the Document tab at top right. Then scroll down to add a featured image in a WordPress post, simply click on “Set Featured Image” link inside the featured image box. This will open theWordPress Media Uploader. You can use that to upload an image from your computer or use an existing image from your media library.