You can easily add a featured image to your post from the same post edit page of your WordPress dashboard. Simply click on “Set Featured Image” link on the right column inside the featured image meta box.
This will open the WordPress Media Uploader. You can use that to upload an image from your computer or use an existing image from your media library. Once you select the image, simply click on Set Featured Image button.
A theme must have the following code in its functions.php file to support the featured image for your post and page.
Display featured image
To display featured images in your theme, you need to edit your templates and add this line of code where you want to display the featured image: