O que você precisa aqui, na minha opinião, para criar pendrive USB inicializável, não é a imagem , mas debian-9.3.0-armhf-netinst.iso
debian-9.3.0-armhf-xfce-CD-1.iso
de acordo com isso:
What is a netinst image?
The netinst CD here is a small CD image that contains just the core Debian installer code and a small core set of text-mode programs (known as "standard" in Debian). To install a desktop or other common software, you'll also need either an Internet connection or some other Debian CD/DVD images.
Espero que isso ajude.