Create Virtual Machine wizard in Hyper-V downloads an image (ubuntu / windows 10) and stores it somewhere (!). When the virtual machine is created, it’s placed correctly in the virtual machine location, as indicated in settings:
But the images that it used to install them are stored on C:\
The “disk cleanup utility” does not seem to see them, which is awkward, because they are stored in the Temp location:
To delete them, simply navigate to %TEMP%
And delete them from there. Voilà, 21GB of my precious SSD disk space recovered.