Everything else seems to be working fine and I have been able to add images to other user profiles in the past. I saw that other users experienced similar issues a few years ago, but I was not able to find a solution on the community forum.
I made a new try with the same image (a 38 KB jpg file) and everything is working fine now. Perhaps there were connectivity issues this morning (even if I was able to listen to music with the Roon client).