Click on the image, then click on the "Edit Background" link you see on the bottom of the section. It looks like you may have already found that, though!
It seems like there's some confusion about the site, maybe? When I log into the editor I see a different image on the blog page and I think that's why Adam thought you figured it out. Can you please confirm the site title and page name?
Click on the section in the editor, then click on "Edit background" in the menu that popups. You should see replace and edit options for the image after that.
