I don't think the OP intends to use an external drive.
You can create a share in Windows and export it over the network. Then you can mount it on Linux and write directly to it with rsync.
Rsync will have a problem making an image backup to NTFS filesystems because some Unix primitives like symbolic links do not have NTFS equivalents. I use the "-L" option to rsync to write copies of the linked files rather than the links themselves. This makes the backup bigger than it should be, of course.
The other, less convenient option is to write a tarball of the system on the Windows machine.
That preserves the structure of the Linux filesystem, but it can take a while to extract one or two files from the backup tarball.
sudo tar cjvpf /path/to/windows/share/backup.tar.bz2 . > /var/log/backup