Add logo above the header image

I’d like to replace the header (site title) text with a logo and also have the header image below that. In other words, I don’t want to use the header image for the logo becauese I’d like to use that for a collage. I saw a response to someone else who wanted to do the same thing and the theme author mentioned that this was a feature of Gridiculous Pro. I have Gridiculous Pro through WordPress.com but I don’t see a way to to accomplish this. Can you tell me how to do this?