Hello Christian,
If you're using document template for PDF generation you need to enable header in properties tab and place the image inside that, I have attached the image below for enabling Header.
Hi Christhian,
If you know how many images fit per page you could render the header multiple times or you can try to use the example from: https://www.geeksforgeeks.org/how-to-print-header-and-footer-on-every-printed-page-of-a-document-in-css/ I have not tried this myself on the PDF doc generator from Mendix, but as it tries to essentially print an HTML page I expect it to work.
Cheers,
Mitchel