1. Open up your Post/Page in the editing screen
2. Locate the Featured Image meta box (usually on the right hand side of the screen)
3. Click on the link Set Featured Image
4. Click to choose an image from the Media Library or upload a new image on the Upload File tab
5. Click the Set Featured Image button

What are Featured Images?

Featured Images are a single image associated with a Post or Page, think of a Featured Image as an image that represents your Post or Page.

Where do Featured Images get displayed?

How your Featured Image is displayed on your WordPress website largely depends on the current theme your site is using.

Some WordPress magazine style themes use smaller thumbnail versions of the Featured Images on the category/archive pages alongside summaries or excerpts, and clicking the Featured Image will lead you through to the single post view with a larger version of the image automatically added.

The size and positioning of the Featured Image will depend on your current theme, any associated settings built into the theme and the size and aspect ratio of the original image.