hey problem here

I need help. so the photos from my iPhone 5S, when connected to the iMac, will automatically sync and the photos from my iPhone 5S can be seen on the iPhoto on iMac. how do I stop this ? I don't want anyone to check my photos when using yhe iMac. I want privacy. I want to set a lock on the iPhone to prevent the pictures from being synced and connected to iPhoto.