As you mentioned that you have code working for Chrome already, it's better to post it, in order to show what exactly you are after.
However, here is how to take screenshot using PhantomJSDriver in C# in general:
var driver = new PhantomJSDriver();
driver.Manage().Window.Maximize(); // optional
driver.Navigate().GoToUrl("http://stackoverflow.com");
driver.TakeScreenshot().SaveAsFile("screenshot.png", ImageFormat.Png);
driver.Quit();
Note that you need to reference WebDriver.Support.dll and System.Drawing in your project.