nightwatch.conf.js
function imgpath (browser) {
  var a = browser.options.desiredCapabilities;
  var meta = [a.platform];
  meta.push(a.browserName ? a.browserName : 'any');
  meta.push(a.version ? a.version : 'any');
  meta.push(a.name); // this is the test filename so always exists.
  var metadata = meta.join('~').toLowerCase().replace(/ /g, '');
  return SCREENSHOT_PATH + metadata + '_' + padLeft(FILECOUNT++) + '_';
}
function imgpath (browser) {
  var a = browser.options.desiredCapabilities;
  var meta = [a.platform];
  meta.push(a.browserName ? a.browserName : 'any');
  meta.push(a.version ? a.version : 'any');
  meta.push(a.name); // this is the test filename so always exists.
  var metadata = meta.join('~').toLowerCase().replace(/ /g, '');
  return SCREENSHOT_PATH + metadata + '_' + padLeft(FILECOUNT++) + '_';
}
The default is to save screenshots to the root of your project even though
there is a screenshots path in the config object above! ... so we need a
function that returns the correct path for storing our screenshots.
While we're at it, we are adding some meta-data to the filename, specifically
the Platform/Browser where the test was run and the test (file) name.