Are you backing up from iTunes or iCloud? If you're deleting them from your device...you should be able to sync and save your current back up.
This sounds like you have gotten them from a sync on your computer. If you have iTunes sync photos to your phone, you will not be allowed to delete them. to fix this, open iTunes on your computer and plug in your phone. Go to the "photos" tab and deselect "sync photos." Then apply and sync your phone. It should delete anything that isn't on the camera roll that could not be deleted before.