By "back them up" what do you mean? Are you talking about an iCloud backup? iTunes backup?
Or... are you talking about what I think you are? Did you use open the camera roll in Windows and copy your camera roll to you computer?
If you did that, then you can't copy it back to the camera roll. The iPhone won't let you... camera roll access from a computer is 1 way. You can copy FROM it but not copy TO it.
But even then... you have your important pics backed up.... why do they need to be stored in your camera roll? Why not keep them stored on your computer, and if you need access to them from your phone, sync them into the Photos app with iTunes.