Thanks Juggledad. OK, I've found a great plug-in that meets my needs (th0th's Quotes) however, being a novice at this stuff, I'm not clear on how to add the widget code or shortcode in place of %image. Everything I try just comes back as code.

BTW, I would prefer to overlay the widget/shortcode, centered, over the header image, not replace the image, but for now I'm happy just to see this method work. See below example.

Thanks in advance,
Lars