Follow these steps to add images to be used in printing in the header

1. Store images in a SharePoint library (site assets for example).

2. In each item, have a Url Hyperlink column, configured to be rendered as an image, each item should have a value linking to its image.

3. Copy the URL of the desired Image item that was uploaded to a Document Library and Paste it in the Main list form where the Hyperlink column field is within the pertaining items.

4. In the Print app header settings, add the Hyperlink Column as a token in the print header.

Result: This will allow you to print the separate images as headers according to the item printed.

If you have any other questions, please let me know.