Chance header picture to page pictures on single pages

Hi, I installed a header picture. But I like to change the picture shown on the top whenever I set an individual picture for that page. On one of my websites it works perfect, but I don't know what I did to get it work. This one works perfect: http://de.lyciasail.com/bucht-von-kekova/ But on the english website, which is seperated, I don't find a way to do it like this. It always looks like this: http://en.lyciasail.com/kekova-bay/ Where can I say "chance header picture to page picture (Beitragsbild in German) for single pages"? Thanks for help! Lycia