My own photo in the header field

Hello
It usually works fine but I now cannot get my own photo to load in the header area of a post. Photos will insert into the text ok but not to the header area. I don’t want to change the front page header, just the headers relating to each post once they’ve been opened.
I’ve tried ‘set header image’ but that just puts the photo on the lower left hand side of the edit page.
Any clues? Thanks