Hi Christian

I'm certainly no expert, but I store my custom header images in src/course/en/images/ so you want to target ../../course/en/assets/ i.e.,

background-image: url('../../course/en/assets/my_image.jpg');

Note that the headers won't display on previewing and you'll need to add your custom images to this folder in the course zip file every time you export. Alternatively I guess you could upload the images in the AT, add the image to a page and hide it, then discover the unique filename generated on building and exporting the course. Finally you'd add that filename to your custom LESS. I find it more convenient to just drop my header images (with human readable filenames) into the archive after the fact.

Hope this helps