Featured images allow you to control what images appear in the slider, as well as on the homepage and archive page. Before publishing a new post, you need to set up the featured image. Here are step by step instructions:
When you create a new post, you will find the Featured Image option to the right side of the editor. Click on the ‘Choose a featured image‘ button.
Upload an image which you want to set as the featured image, or choose an existing image from the Media Library. Click on ‘Use as featured image‘ link.