Said and done.
My simple way of skinning the cat is: copy your avatar (as .png, I think) into a file named ~/.face
Done, works 
Edit / bonus tip:
To use the picture also for sddm (but this was not part of the bug), you can check the
[Theme]
FacesDir=
directory (default is /usr/share/sddm/faces) in /etc/sddm.conf and copy there the avatar file as <username>.face.icon (or of course you can create e.g. /etc/sddm.conf.d/avatar.conf with a different folder)
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