Unfortunately, moving the header image is a bit beyond Yumblog’s layout, which would require customization that cannot be done on WordPress.com.
The Custom Design upgrade allows you to edit fonts and CSS only. You can change the appearance of a theme, but not its HTML markup, PHP code, or content. If you require this functionality, please consider moving your blog to a web host that supports theme code modifications, suitable hosts can be found here: http://wordpress.org/hosting. Read more about why here: http://support.wordpress.com/com-vs-org.