Typo3 template override
===

All,

I am working on a simple and straightforward site with Typo3 CMS. Using a template, all of the pages have the same banner image displayed at the top of the page. I want to change this image on one of the pages to a different image. What is the best way to go about this? 

The image is being loaded from a template extension called 'Banner' which looks like this:

    temp.banner = COBJ_ARRAY
    temp.banner {
    # Menu 1 cObject
    20 = IMAGE
    20.file.import = upload/pics
    20.file = fileadmin/template/main/template_1_FILES/{$banner_img}
    }

I then tried to make a template extension on the one page which looked like this:
    
    temp.banner {
    20.file = fileadmin/template/main/template_1_FILES/otherimage.jpg
    ]

But this did not change the image. The original image was still displayed without any errors or sign that the extension did anything.