How To Save Canvas As An Image With canvas.toDataURL()?

I’m currently building a HTML5 web app/Phonegap native app and I can’t seem to figure out how to save my canvas as an image with canvas.toDataURL(). Can somebody help me out?

Here’s the code, what’s wrong with it?

//My canvas was named “canvasSignature”

JavaScript:

---

function putImage()
{
  var canvas1 = document.getElementById("canvasSignature");        
  if (canvas1.getContext) {
     var ctx = canvas1.getContext("2d");                
     var myImage = canvas1.toDataURL("image/png");      
  }
  var imageElement = document.getElementById("MyPix");  
  imageElement.src = myImage;                           

}  
HTML5:

<div id="createPNGButton">
    <button onclick="putImage()">Save as Image</button>        
</div>