Jerryf's answer is fine, except for one flaw.
The onload event should be set before the src. Sometimes the src can be loaded instantly and never fire the onload event.
(Like Totty.js pointed out.)
var canvas = document.getElementById("c");
var ctx = canvas.getContext("2d");
var image = new Image();
image.onload = function() {
ctx.drawImage(image, 0, 0);
};
image.src = "data:image/ png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAIAAAACDbGyAAAAAXNSR0IArs4c6QAAAAlwSFlzAAALEwAACxMBAJqcGAAAAAd0SU1FB9oMCRUiMrIBQVkAAAAZdEVYdENvbW1lbnQAQ3JlYXRlZCB3aXRoIEdJTVBXgQ4XAAAADElEQVQI12NgoC4AAABQAAEiE+h1AAAAAElFTkSuQmCC";