Hi Axel,
Page header has already been replaced by Header Element.
https://docs.generatepress.com/article/header-element-overview/
Here’s an article about how to migrate page header to header element.
https://docs.generatepress.com/article/moving-hooks-page-headers-to-elements/
In your case, the steps would be:
1. Go to Appearance > Elements, create a new Header Element, add this Html to the content field:
<img src="your-image-link">
.
2. Set Navigation location to Above header.
https://www.screencast.com/t/XHzUOsssceF
3. Choose where you’d like it to show in Display rule.
Let me know if you need further assistance π