In addition to the post from Josh Odgers it seems it is not well known how to upload an ISO/Image directly from a NFS share to the image service. To achieve this you can leverage the “From URL” field in the PRISM interface.
The format for anonymous nfs access is:
nfs://IP-or-DNS/share/subfolders/isofilename
If user and password is required:
nfs://user:password@IP-or-DNS/share/subfolders/isofilename
Example: