Why not just do something like the below when you need to do that?

  var img = new Image();
  img.onload = function(){
       //draw to canvas
  };
  img.src = "image.png";

Could be a stupid answer, or possibly it'd be better to do it your way?( Maybe I can learn something :) )

All major browsers will now do this for you, with data: urls. Take a look at the examples here: http://en.wikipedia.org/wiki/Data_URI_scheme

This may be helpful:

https://developer.mozilla.org/en/Canvas_tutorial/Using_image...