Right, the blog now showing uses the new Vigilance theme. Until an hour ago, it was using the Cutline theme and had a custom header photo. I thought I could play around with the new theme and activated it; as soon as I did the header photo disappeared, and it really is not in my media library. Should be, you would think, but is not.
In the meantime I found an old blog page I webarchived on my computer one time when I tried out various header styles and then saved the page so I could remind myself of exactly what they looked like.(I put up a post with various headers, webarchived it, then deleted the post) It has my old photo on it so I have copied it and will try to use it with Vigilance, although I also will have to reposition the title---vigilance wants to plaster it right across the photo space.
Having found the copy, at least I should be able to go back to the old Cutline theme and recreate the previous appearance.