If you're making a mobile game you probably noticed Apple requires screenshots at quite a few different resolutions with different aspect ratios. Even if you're not working in mobile there's a decent chance you want to be able to do this. Check the code below for one way to do it automatically.ScreenshotUtil.cs
To use setup a shortcut key in code to call the function ScreenshotUtil.TakeMobileScreenshots(). The log will show the directory / name and size of all the screenshots saved.
This code was tested in Unity 4.6. It works in editor or standalone game. One of the tougher things I ran into was getting the new Unity UI to update automatically when the render texture size and aspect changed. I just fooled around with it for an hour so it might not handle all cases.