[TYPO3-ttnews] Image from tt_news as header image

[Ralf Heydenreich]

Hi all,
I want to achieve the following: My web page consists of a variable
header image. I'm using Templavoilà for the page template. The header
image is defined as data content in this template (lib.header field).

Now, on single view of a news (from tt_news) I want to exchange this
header image with an image from Media content. How can I get this?
Thanks in advance.

Ralf.