I recently changed my them to Twenty-Twelve, added a tiled background image and a header image. On two occasions now while I've been editing a post (once) and a page (once) to find that the header image vanished from the blog. It still appears in the Media library but in Appearance -> Header the radio button is present but no image is visible. I find I have to upload it again to fix the problem. Any ideas why this is and how to avoid it?
There is supposed to be a banner image with L-R the moon, a fog filled valley and a set set.
The blog I need help with is richardleonard.wordpress.com.