Hey.

I have instances with a volume that were once created from some image.

Now the volume's OS was upgraded over time to the respective current
releases, while the image is long obsolete and just uses up space.

Is there a way to remove those images? It seems normal commands don't
allow me, as long as there are volumes which were created from them.

  

Instance disks are changes over time from a baseline. What this means is, you can't delete the origin without destroying all of its descendants. What you can do is set it to "hidden" so it won't show up in the default image list. You'll still be able to explicitly look for it though, and instances that depend on it can find it as well. 

Thanks,
Chris.