When walking through the setup process on a new iOS device, sometimes New Tech Fever causes you to inadvertently enable or overlook a feature in your haste to play with a new device. One feature that it's easy to turn on and forget about is Photo Stream, which will sync all your photos to your iCloud account. Since those photos don't count against your quota, you won't get alerts about your storage being full, and there's rarely an error that reminds you the feature is even enabled. Here's how to check if Photo Syncing is enabled, and if you wish, how to turn it off:
On your iOS 7 device, go to Settings > iCloud > Photos. (Using iOS 6 it's Settings > iCloud > Photo Stream)
Check the slider there for My Photo Stream, and see if it's on or off.
Here's where the magic happens: Enable or disable My Photo Stream as necessary.
NOTE: This is different from the Photo Sharing option, which allows you to share a photo stream with others or subscribe to a stream shared by someone else.