[MickMcGeough]

- bloginfo('template_directory') echoes the current theme's root directory (usually something like "{webroot}/wp-content/themes/{theme_name}")
- you should have a directory called "banners" there.
- each post that uses this template should have a custom field called "page_banner". The value of this field should be the filename of the image you want (just the name, not the path).

ETA: You shouldn't need to add a new template to add a new page, unless none of the layouts of the existing templates does what you need. Basic sites shouldn't need more than a few templates.