Delete The Photos Already On The Cloud
Go to "Settings." Go to "iCloud." Go to "Storage & Backup." Go to "Manage Storage." Choose your phone. Turn off "Camera Roll."
Of course, this small levee doesn't protect against passcode breaches, other individuals having the pics (assuming they weren't just for your own enjoyment), Chinese government hacker gymnastics, or people taking pics of your phone over your shoulder with their phone. And if you want to have photos up on iCloud? Get two-step verification to keep them just a little bit safer.
Still, the above steps will help keep those precious megabytes (or gigabytes, you sly dog) in your possession, not someone else's.
Ethan Wolff-Mann is an editor at Supercompressor. Stop trying to get into his phone. Follow him on Twitter @ewolffmann.