What is the resolution and image size to use for high-fidelity UX prototypes? I am supply images to someone doing a UX design for smartphones-iphone and android, tablets and desktop--I am wondering if 72 resolution is good enough and an image that is what pixels (or inches) by what pixels (or inches).
Resolution should match the target device whenever possible, both for prototypes and production. For instance, images for an apple retina mobile screen should match its @2x/@3x guidelines, but also keeping in mind these images would not be appropriate for a television or a watch.
In more plain terms if the device is 700px wide your image should also reflect that width. Usually when it comes to resolution 72ppi is pretty much the standard when it comes to the web, unless in cases of apple retina devices.
Since its only a prototype the resolution does not matter. For web just use 1920*1080, for mobile Apps just use the dimensions of the iPhone 6/7 (375x667).
If you hand the finished design to the developers you should just make sure to export every asset as 2x for retina displays so your icons and images look sharp on every device.
Ps.: the 72ppi myth should die already