[swizz-ly]

Ok I found it out. On Ubuntu based distros you first have to install the corresponding package:

bash:
$ sudo apt install kodi-imagedecoder-raw

then the add-on appears in the "My addons" as mentioned above.
Unfortunately on raspbian, there is no such package, or I have not found yet.

[Karellen]

If I browse the Image Add-On repositories, I can't find any RAW related Add-Ons.

Add-ons > Box Icon > Install from repository > Image decoder and install whichever one is appropriate. Usually libraw image decoder will decode standard RAW format

As for the advancedsettings.xml, the requirement to add your image extensions is no longer required and is handled by the above add-on. FYI, the advancedsettings.xml does not exist by default, you need to create it the first time.