Featured images are designed to enhance the appeal of your blog’s homepage, individual posts or individual pages.
Featured images are theme dependent.
How featured images are used
Not all themes support featured images and how the image is displayed on themes that support featured image varies.
Here’s some of the different ways feature images are used on themes:
Blog post homepage
Featured images can be used on a range of different ways on the blog post homepage.
Featured images can appear as thumbnail image that accompany a blog post title on a blog’s homepage. A post excerpt might be displayed with the image.
Themes like Broadwalk, Cubic, Fifteen, Fontfolio, Gridster-Lite, Hatch, Hitchcock, Intro, Klean, Magazino, Market, Oria, Pictorio, PR News, Rowling, Snaps, Snapshot, Spun display images only on the blog post homepage.

While themes like Briks, Celebrate, Coller, Dellow, Freak, Griffin, iTek, Oblique, Rectangulum, Surfarama displays thumbnail image with the post excerpt on the blog post homepage.

Grid based themes designed to display large images on their blog post homepage are popular for photoblogging. Some of these themes will pull the first image in the post onto the blog post homepage if a featured image isn’t set while other themes may need the featured image to be set to display an image.
Featured images may appear as a background behind the post title on the blog post homepage. A post excerpt might be displayed with the image. Themes that do this include Intergalactic, Pique and Radcliffe.

Post page
Featured image can appear above or below the post title on an individual post or an individual page. Occasionally you may have situations where you want images displayed above a post title and this is when you might use this type of theme.
Some themes that display the image thumbnail on the blog post homepage display the featured image above or below the post on the individual post page. For example, the theme Boardwalk displays featured images only on the blog post homepage and the featured image above the post title on the post page.
Header
Featured image can appear in the header on an individual post or an individual page. Themes that do this include Adler, Canard, Clearly, Coherent, Cubic, Ecto, Fortunato, Gazette, Harmonic, Landscape, Lovecraft, Lyretail, Pictorico, Radcliffe, Rectangulum and Resonar.
This is handy if you want to control which image appears in the header on different pages on your blog.
Sidebar
Featured image can appear in the sidebar on an individual post or individual page. Themes that do this include Bushwick, Cruzy, Dyad and Eryn.

Set Featured Image
Themes that support featured images have a Featured image option in the right sidebar of the post editor and page editor.
How you set a featured image depends on if you are using the classic editor or the block editor.
Block Editor
The following section explains how to set a featured image using the block editor. If you are using the classic editor, refer to publish new post using classic editor section.
You set a featured image on a post or page using the block editor as follows:
1. Click on Post or Page to access the Document Settings in the sidebar of the post/page editor.

If you don’t see the Document settings, click on the cog next to Publish.

2. Click on Set Featured Image in the Featured Image module.

3. This launches the Featured Image window where you can upload an image or select an image from your media library.
4. Once the image has uploaded, or you’ve selected the image you want to use, click on Set Featured Image.

You should see the image displayed in the featured image module.

5. Click Update or Publish to save your changes.
Classic Editor
The following section explains how to set a featured image using the classic editor. If you are using the block editor, refer to how to set a featured image using the block editor section.
You set a featured image on a post or page using the classic editor as follows:
1. Scroll down to Featured image module.

2. Click on Set Featured Image.

3. This launches the Featured Image window where you can upload an image or select an image from your media library.
4. Once the image has uploaded, or you’ve selected the image you want to use, click on Set Featured Image.

You should see the image displayed in the featured image module.

5. Click Update or Publish to save your changes.
Remove Featured Image
To remove or change a featured image, simply re-open the post or page editor and click on Remove featured image.

Once the featured image has been removed you’ll see the option to Set featured image.