Casper.js change of file name to show url and saved to root screenshot directory ref: http://net.tutsplus.com/tutorials/javascript-ajax/responsive-screenshots-with-casper/

casperscreens.js

var casper = require("casper").create(),
    viewportSizes = [
    [320,480],
    [320,568],
    [600,1024],
    [1024,768],
    [1280,800],
    [1440,900]
],
    url = casper.cli.args[0],
    saveDir = ('');
    saveName = url.replace(/[^a-zA-Z0-9]/gi, '-').replace(/^https?-+/, '');

casper.start();

casper.each(viewportSizes, function(self, viewportSize, i) {
    // set two vars for the viewport height and width as we loop through each item in the viewport array
    var width = viewportSize[0],
        height = viewportSize[1];
    //give some time for the page to load
    casper.wait(5000, function() {
        //set the viewport to the desired height and width
        this.viewport(width, height);
        casper.thenOpen(url, function() {
            this.echo('Opening at ' + width + ' x ' + height);
            //Set up two vars, one for the fullpage save, one for the actual viewport save
            var FPfilename = saveDir + saveName + '-fullpage-' + width + ".png";
            var ACfilename = saveDir + saveName + '-viewport-' + width + '-' + height + ".png";
            //Capture selector captures the whole body
            this.captureSelector(FPfilename, 'body');
            //capture snaps a defined selection of the page
            this.capture(ACfilename,{top: 0,left: 0,width: width, height: height});
            this.echo('snapshot taken');
        });
    });
});

casper.run(function() {
    this.echo('Finished captures for ' + url).exit();
});