Have you switched to a new theme only to find its layout broken due to missing featured images on your site? Many of us forget to set a featured image for our posts. You can always download a plugin to set one for each post automatically or use Featured Image Reminder to remember to set a featured image for each new article.
Featured Image Reminder is pretty simple to use. Once you have installed it, you get a message when you try to publish a post without a featured image. You are going to have a hard time missing the message, which is helpful. This is a free plugin.