Q: Photo download from iPhone to iMac

Historically the way that you moved photos from a computer to an iPhone was via iTunes photo sync: Sync photos from your computer to your iOS device using iTunes - Apple Support

That is still a supported method.

Other methods now available include:

iCloud Photo Library -- all of your photos will automatically be on both devices (assuming you turn it on for both devices) and on iCloud. iCloud Photo Library FAQ - Apple Support

via PhotoStream: My Photo Stream FAQ - Apple Support

via a 3rd party app such as http://photosync-app.com

via 3rd party services such as Dropbox