gistfile1.js

var deferred = {},

if (data.picture) {
	deferred.picture = webservices.image(data['picture']);
}

if (data.screen) {
	deferred.screen = webservices.image(data['screen']);
}

if (data.screens) {
	data.screens.forEach(function() {
		???
	})
}

when.keys(deferred).then(
	function(deferred) {
		console.log(deferred.picture);
		console.log(deferred.screen);
	}
);