Sorry for anyone attempting to use the new Unique Headers plugin. It seems I launched the plugin with a rather annoying bug in the posts/pages implementation of it. It seems to work fine with categories, but the posts/pages are not allowing the new image to be added.
I'll have that fixed within the next day or two. I can't get to it this evening unfortunately, but rest assured I am aware of it and must fix it very soon. This critical functionality on a site I'm working on right now, so I guarantee it will be fixed as soon as I can get to it.