Q: how to transfer picture from mac to iphone6

There are a number of ways, including:

via USB cable and iTunes: Sync photos from your computer to your iOS device using iTunes - Apple Support

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

via Photo Stream: My Photo Stream FAQ - Apple Support

via Airdrop if your Mac is a 2012 or later model and if it is running Yosemite: Mac Basics: AirDrop lets you send files from your Mac to nearby Macs and iOS devices - Apple Support

or via email to yourself