I already found the solution:
in the media settings, each images is assigned with a language. If you upload an image, it is automatically assigned with the standard language.
If you switch to 2nd language, polylang doesnt display these images because they are assigned to the other language.
the solution is to upload a duplicate of each image and in the media library select the 2nd language.
Then, in the Customize options of Azuma, upload also these header images.
Hope it helps somebody.