If you haven't used [grunt] before, be sure to check out the [Getting Started] guide, as it explains how to create a [gruntfile][Getting Started] as well as install and use grunt plugins. Once you're familiar with that process, install this plugin with this command:
npm install --save-dev grunt-phantom-rasterize
See the Gruntfile in this repo for a full example.
Example Generating Icons for an iOS Application
Tip: the grunt-newer module might come in handy if you have a large number of files.