The Featured Image in WordPress can be better described as the post thumbnails. The Image which is visible to you at top when you read a blog article, which describes the topic of the post. It is the representative image of your posts and pages.

The most ideal size of the WordPress Featured image is 1200 x 628 pixels. And these are the standard dimensions for a featured image in any blog theme. Anyone can simply change or add the featured image in the post. 

While editing a post in the WordPress, scroll down and you’ll see a button saying “Set Featured Image” and click on the button to select the image from the Media settings. You can also upload the image in your WordPress media and then set it as a featured image. 

If you can not see the Set featured image button then you can check in the admin panel whether it is disabled an the currently installed theme support the featured image functionality.

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How to Add Featured image in WordPress – Tutorial for beginners