Image on the top Category

[quattrossa]

Hello,
first of all, I hope i m not breaking any rules..
i m try to work on a new template to understeand how everithing it works.I used the Chrmo in 3.4 as a basis template.

I wanted to add a image on the top category.
I found in forum a string and i change the main.tpl.
I did it and it works fine, example hier:
http://www.private-homepage-verzeichnis.de/

the problem is, that now if you click on a top category, like for exemple the first one:
"aubildung Beruf Karriere" (doesnt need to translate to understeand the error i get ;))
you come to the follow page:
http://www.private-homepage-verzeichnis.de/ausbildung-beruf-karriere/

well, hier the Categori ( sub category of the top) are displayed not anymore in the right position, becouse, the background image that i put on the top category, it will appear hier too as a white space.
and so it shown on all sub category, and the sub sub one, and so on...

I would like to have the image only on the top category shown.

anyone has a int, what i can do to show the image only on the first page?

thank you

Antonio