Quote:
Hi there, my header image is no longer showing on my site
Your header image is located at http://www.lynnclarke.net/wp-content/uploads/2014/04/cropped-hdf.png
but that is not a valid URL.
As for the customizer, it is possibly a plugin conflict. Try disabling your plugins to see if the problem resolves itself.
Have you edited an core files (WordPress, themes, plugins?)