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!
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I don't know what else to suggest, but I've passed this on the HQ staff.
The old editor appearing is a bug, but to make the image change "stick", don't hit "save", hit "publish" again.
That happened to me over a year ago using the old editor.
I deleted the main image and it the ible took the second image in line,
Then I added the that image from my library to sit elsewhere in my ible.
I'm guessing there is an update every week late Saturday. So try again Mon AM...
BTW NEVER ask HQ to delete things for you ! .... Yes HQ deleted what I asked BUT they also put huge gaping holes in several ibles I was proud of.. Bah !