Hi There,Sorry to see you're having trouble changing the main images on some of your Instructables. The Instructables you're trying to edit pre-dated the editor switchover, and were all "Photo Instructables" -- Instructables that let you share photos of your finished project without having to write "how to" steps at length.When we switched to the new editor, we also dropped the distinction between "Photo" Instructables and standard "Step by Step" Instructables. But unfortunately our servers aren't able to convert between the types. Because there are certain differences between the types of Instructables, we opted to keep the old editor in place so that you'd be able to edit Instructables created in the old styles.The old editor always had the issues you're seeing with cover images, though, so unfortunately there's no easy way to change the images. You can set a cover image once, but then there's no way to change it.If you have specific Instructables you'd like to change the cover images for, please go ahead and send me a private message; I'll try to do one or two for you manually.Thanks for being an involved member!