Debian will not ship non-free software in its main repos / official installers. The unofficial images are exactly that, created outside the rules of the project to avoid this prohibition. But, you don't have to trust them.
You can download and place the non-free firmware files on a thumbdrive yourself, and the official installer can use them. It will stop and prompt you to insert the thumb drive if it is necessary to complete the installation.
If you e.g., netinstall over a wired ethernet connection, it is rarely a problem. But, all 802.11ac and above wifi chipsets require a non-free firmware. You can always add non-free to your sources.list and install nonfree firmware after installation for any devices other than those required for installation, i.e., your network adapter (and in maybe a super rare case for a desktop install, your disk controller).
You can download and place the non-free firmware files on a thumbdrive yourself, and the official installer can use them. It will stop and prompt you to insert the thumb drive if it is necessary to complete the installation.
https://wiki.debian.org/Firmware
If you e.g., netinstall over a wired ethernet connection, it is rarely a problem. But, all 802.11ac and above wifi chipsets require a non-free firmware. You can always add non-free to your sources.list and install nonfree firmware after installation for any devices other than those required for installation, i.e., your network adapter (and in maybe a super rare case for a desktop install, your disk controller).