What are you using to backup? If you're using iCloud then you have 5GB to backup all devices connected to the iCloud account. It happens very often that because of the camera roll there isn't enough space for the iCloud backup to complete. Go into Settings, iCloud, Storage & Backup. Then tap Manage Storage, tap the device and it will show you everything being backed up under Backup Options along with the size. If the Camera Roll is huge then toggle it off to remove it from the backup. Just be aware that the pictures will not be backed up so if you need to restore your phone for any reason you will not get the pictures back. So use either online storage or sync the pictures to your computer to make sure you have them copied somewhere.