How to sync images between iPhone and Mac in a local network
I use an app called LiveView. You run the server on your Mac, and the app on you Phone. It creates an iPhone-sized area on your Mac's screen, and broadcasts whatever is inside it to your phone.

I work in Illustrator when I'm doing this, but that doesn't matter. Here's the best system I've worked out:


Create a second window of whatever your working on (Photoshop and
Illustrator both do this).
Zoom that window so that my artboard fits the window perfectly (in
Illustrator this will be 100% or 50% depending on whether you're
testing on a retina device).
Position the LiveView window over the duplicate window (I toss this
onto a second monitor, or tile the windows if you only have one
monitor).
Work in the original Window.


You may now zoom in and out of the window you are working on, all while enjoying a perfect streaming vision of what it looks like on your phone the whole time.

Once I need to take the app to the device to show people, I use a service called InVision (invisionapp.com) to create touchable prototypes.