Setting up a featured image for a post or page is quick and easy; to do so, locate the featured image box* on your editor page, and select Set Featured Image. When you’ve done so, it will bring up a page from where you will be able to drag-and-drop files, or select them straight off of your hard drive, to select the wanted image.
You can also choose from images that are in your media library, if you already have it added there.
After you have selected the image, you will have options available to either edit the image or delete it if needed. Once you have finished selecting the right image and editing it to perfection, you can preview your changes or go ahead and publish the page/post with the image viewable to the public.
*If you are having difficulties locating this, it should be on the right hand side of the page. You may need to scroll down a bit to find it.