Only two ways I know of:
1- Transfer the picture to your computer and sync it back to the iPhone through iTunes as a separate album from Camera Roll.
2- Send the picture to a Photo Stream album, which you can do right from the iPhone.
In either case you can then delete the picture from Camera Roll without deleting the picture everywhere.
The Photo app doesn't allow that.
Copy the photos in your Camera Roll to your computer. Organize them there. Use iPhoto if you have a Mac or folders if you use a pc. Then sync those new folders to your iPhone.
You can then delete all the stuff in your camera Roll and delete the additional folders you created directly on your iPhone.
Is it me, or is this confusing? (it's likely me!)
Anyway, I use Aperture and have Photo Stream enabled to copy all my photos i take on my phone to my Aperture library. But, i'm guessing if i delete the photo from my phone/camera roll, it will delete it from Aperture. Anyone know this?
So, if i wanted to prevent that from happening, i suppose i should move it from my photo stream and put in a separate album? (I'm confusing myself now! )