I think the need for customization, to give to your blog a personal differentiating touch, is strong and clear.
I was thinking: why not create an additional sheet just for the header? People could place a hosted image (in Flickr, etc), say of 200 by 760 px, without affecting the rest of the template sheets. It could even be 500 by 760 px and that would create an opening screen to the blog.
No need for knowledge of html to do that,you just place your link to your image at Flickr and that`s it.
What about that?