A featured image – also known as a post thumbnail – is an available feature on each post. The featured image can be used in many different ways (this will depend on each theme). It can be used as the header image when viewing a post, it can be the image that appears on top of each post which will serve as the primary image, it can also be used for thumbnail images that accompanies a post when viewing the blog page.
A featured image is set on each post ideally before publishing. You can find the Featured Image Area on the right column of the post editor. You could use any image that is attached to the post or even an image used on other posts. The chosen image doesn’t necessarily have to even be attached so you could set a Featured Image even without having an actual image on the particular post.
Where to set a Featured Image
Click the images below if you have trouble finding the Featured Image Area. It could be hidden by default in some cases. Click the Screen Options on the top right corner of the editor page. Then check the Featured Image checkbox.
Using a Plugin to set a Featured Image
If you already have tons of posts without featured images, you may not want to manually set a featured image for each. In this case, using a plugin to do the job might be ideal. A plugin like this one works nicely auto post thumbnail. You could deactivate the plugin once you’re done or maybe leave it “on” to automatically set a featured image every time you publish a post. Though you won’t be able to choose which image to use as the plugin will automatically set the first image as the featured image.