how do i transfer photos from iPhone 5s to my iMac

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 3rd party cloud services such as Dropbox

via Photo Stream (but only for new photos taken after you have turned on Photo Stream) My Photo Stream FAQ - Apple Support

or if your Mac is running a recent version of Yosemite with Photos you can have all of your photos automatically on both the Mac and the phone as well as iCloud via iCloud Photo Library FAQ - Apple Support