In WordPress, the featured image is the term for a single image that is attached to a given post or page. For older versions of WordPress, this was often known as a Post Thumbnail.
Let’s imagine that you create a post that has several different images. WordPress will then take the first image used and label it the featured image. Alternatively, you can tell WordPress which image you want to be used as the featured image by clicking on the lower right-hand side of your screen (“set featured image”) when you are creating the post.
Note: If you don’t see the “set featured image” option when creating a post, you will need to enable this function using Screen Options located at the top right-hand corner of the WordPress administration screen.
The featured image will then be used every time your post is displayed. On your own WordPress website, your theme will determine how and where the featured image is displayed as well as the size and layout of the image. Other websites (like Facebook) that link to your post will also use the featured image as the image that accompanies the link.
Despite originally being called a “thumbnail,” there are no restrictions on the size of how a featured image will be displayed as that is determined exclusively by your WordPress theme. If you’re unhappy with the way featured images are being displayed on your website, then you need to choose a new theme or make modifications to your existing theme. However, the way that featured images are displayed on third-party websites like Facebook is entirely up to the third-party site.
WordPress only allows you to use static images as a featured image, but there are plugins available that allow you to instead use a video as the featured image.