These are cover photos for the articles. If you want to add a cover pic to your articles, go in to edit the article and then upload an image. You don't have to insert the image anywhere into the article, you can save it as it and the image will show up as the cover pic. If you have a cover photo on, you should be able to change it to something different. Click on article tools and look for "Set Cover Image"
(You have to have some images uploaded for this option) I used my egg article as example as I uploaded images after the conversion, so they are showing up. Click the image you'd like to show up as cover pic and set it.