You can't just put the name of the image, since it is a spot for text. You have to first tell it that it is an image, and then what directory it is in. For example, if you put the image in
includes/templates/YOUR_TEMPLATE/images/, put this:
Code:
define('HEADER_SALES_TEXT', '<img src="includes/templates/YOUR_TEMPLATE/images/banner.gif" alt="" width="" height="" />');