You might be sensibly backing up the photos on you phone so that should you lose your iPhone, or get a new iPhone, you will easily be able to download all your photos to your new device. You might be wise to back these images up to your Mac rather than to the cloud, however.
If you’ve switched on iCloud Backup on your iPhone, the images it holds are sitting in iCloud in the form of a backup archive. If someone gains access to that backup, they will also gain access to all your images and movies.
To stop your iPhone from creating such a backup, tap Settings > iCloud > Storage & Backup and disable the iCloud Backup option (which automatically backs up your camera roll, accounts, documents and settings whenever you plug your iPhone into a power source.
It is possible to keep on backing up to iCloud without backing up your photos though. Tap on Settings > iCloud > Storage & Backup > Manage Storage, then tap on the relevant backup and deselect Camera Roll.
You can also back up your iPhone by connecting it to your computer and backing it up via iTunes.
how to manage your iCloud Back Up settings