I downloaded image "Ubuntu 16.04 64-bit KVM instance with base OS" in Cloudmin Settings -> New System Images, process completed successfully. But I can't user the image because file path in /var/webmin/server-manager/kvm-64-ubuntu16.04-base.img is incorrent - "file=/var/webmin/server-manager/kvm-64-ubuntu16.04-base.im". I set up default image storage location to /virt/images before (in System Storage -> Image Storage Locations), so the right file path should be "file=/virt/images/kvm-64-ubuntu16.04-base.im". I checked downloaded file and it's stored in /virt/images, the correct place.
It seems like image downloading process is ignoring default system image location and do not update file path in configuration file after download.