Q: how   do i download pictures on my iphone 5 back to my mac

There are a number of methods:

via USB cable: Import photos and videos from your iPhone, iPad, or iPod touch to your Mac or Windows PC - Apple Support

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

via Photo Stream (for new photos taken after you turn Photo Stream on): My Photo Stream FAQ - Apple Support

or if you have the new Photos with Yosemite OS X 10.10.3 or later and iOS 8.3 or later on your phone and want all of your photos to be on both the Mac, the iPhone and iCloud: iCloud Photo Library FAQ - Apple Support