Mike Kahn converted one iPhotos library to Photos in OS X. Now, every time he plugs in his iPhone, Photos launches, requiring him to quit and then launch iPhoto. He’d like to revert to iPhoto being the default choice.
Apple likes to hide its browser, mail, and iOS-attached launch preferences in funny places. You set your default browser in Safari, meaning you have to launch Safari to stop launching Safari in the future (Safari > Preferences > General, Set Default Browser); same with Mail and mail (Mail > Preferences > General, Default Email Reader).
Where do you find the find the photo-app launching preference? Not in Photos or iPhoto, as you’d expect! Instead, launch Applications > Image Capture, a very useful utility that can work with iOS devices, inserted SD cards, attached cameras, and networked scanners.
Read more here:: Stop Photos from launching when you plug your iPhone into your Mac