Forgot a Thumbnail While Editing

I was making a new concept page but forgot to add a thumbnail image while creating the page. I've been trying to find a way to add one while editing but cannot figure out how to do it. Does anyone know how to do it or if it is possible?

@chemystery: What page?

http://www.giantbomb.com/forums/delete-combine-requests-34/request-a-fix-for-images-box-art-issues-here-570461/#117