If you want image files, I would use iso instead of .nrg. You can make them using a free tool like DVDFab HD Decrypter. You can backup the entire movie to an iso, or you can just get the main movie and make an iso of it. http://www.dvdfab.com/free.htm
DVDDecrypter can also make iso files in ISO Read Mode, but it is not effective against many newer forms of protection put on dvds. Use it in combination with AnyDVD and it will work for these newer disks.
A third way is to break encryption and put the dvd on the hard drive with whichever program is your favorite decrypter/ripper, then make the iso with ImgBurn.
The only real reason to use iso over nrg is that nrg is a proprietary format. Iso’s are just more universally supported.