Many WordPress themes, especially magazine-style themes show thumbnails for your posts. Post thumbnails was a feature introduced with WordPress version 2.9. Themes that show the corresponding thumbnail along with every post need the posts to have a featured image. If your posts do not have a featured image, the space will show blank. One way to make sure all posts have thumbnails is to add featured images to all the posts manually:
This may not be feasible if you have a large number of posts without featured images. To make this easier, you can use a plugin called ‘Auto Post Thumbnail’ that automatically does the task for you. This plugin takes the images from the post itself, and sets it as the featured image for the post. Here’s a snapshot of the plugin in action:
Auto Post Thumbnail Plugin
Further, for posts without any images, you can add a featured image manually. The links for the plugin are given below:
Auto Post Thumbnail on WordPress: Link
Plugin Page on Developer’s Site: Link