Now you can automatically set all first images of published post, pages and other custom post types as featured.
Set All First Images As Featured is a free WordPress plugin that allows you to run a process of creating post thumbnails i.e. featured images. It also provides you an option to overwrite featured post images with the first images inside post.
Start by installing and activating this plugin in your WordPress site. Upon activation visit your admin area dashboard Tools -> First Image As Featured page and select a post type from the drop down menu.
Now select the option: you can choose the post type you want to run the plugin on, you can overwrite the already selected featured images and select From / To date. Finally, when you’re ready, click the ‘Start’ button.