kobra:
If I am understanding what you want...Try creating the image that you want displayed that you are calling a "Banner" and in the admin > catalog > categories/products > edit a category > attach the image to the category in the "Category Image:" function box.
This will display the image only when the category is selected. You will have to adjust the image size to display in admin > config > images > Category Icon Image Width - Product Info Pages and Category Icon Image Height - Product Info Pages - as this is intended to be an "Icon" image
See if this does not satisfy what you seek
Thanks for your answer but I really want a real banner to be shown on specific categories pages. I though that would have been actual thought already in the banner management structure but atleast I do not find such option.
FOR EXAMPLE: Banner is an advertisement and if one advertiser is interested in putting his banner of printers that are related products of my category of "printing material", he is not necessarily interested in putting his banner for the page of selling socks in the same shop. On the other hand I may have another banner advertiser who is selling shoes and is interested in putting his banner on the page of socks indeed.
Maybe this example above clarified my need for the banner. ;)
Elli