Deafult block in a template. Edit it on a specific page

[dosch]

I made a single page template containing a big image on top with the post title in it.
Now I want to change this header image on every page, but let the opther elements stay the same.

How do I best do this? I can't find a way to edit the image for just one page, only to edit it on the Single Page template.

Thnx!