How do I stop iPhoto from opening automatically every time I connect my iPhone to my mac?

Open Image Capture, go to Preferences and in the ‘When a camera is connected, open:’ dropdown select ‘None’. Unfortunately, this means that iPhoto won’t open when you plug in any camera anymore. Although in my opinion, that’s a good thing.

To add to @yannick, iPhoto will see the camera or it should regardless of the above setting, so there’s no real danger in setting this preference.

Check out my question in siblings. Fourth from the top.