The camera roll is temporary storage for photos you have recently taken, before you can off-load them to your computer. So you can't add photos to it from your Mac.
It would be better to set the iPhone's Photos tab in iTunes (in the row of tabs that starts with Summary when the iPhone is selected in iTunes) to include that folder on your Mac with the pictures you want on the iPhone. Check the box for Sync Photos from and select that folder with the photos. Those photos should sync to the iPhone, and they should be viewable in the Photos app (on the iPhone).