You’ll find that most blogs that use WordPress as their blogging platform use featured images on their posts. Featured images are what you usually see when you share a post on social media (the thumbnails that appear when sharing on Facebook or Google+), related posts widgets and in the post itself.
It’s really simple to add the featured image functionality, if you don’t have it already. You just have to modify one theme file and you’re done.
Open your current theme’s functions.php file
The functions file contains all the snippets necessary to make sure the theme runs correctly and to add any custom modifications to your theme.
Add the next snippet to your functions.php file
By adding that code, you should have the featured image functionality. When you write a post, you should be able to see the ‘Featured Image’ box to your right. With a prompt to add an image.