You have an existing image (graphic, icon, photo, etc.) in an external file that you want to put into a canvas element, so that you can then manipulate or embellish it with canvas API drawing commands.

First, either load the image in question using an img tag in your markup and get a reference to it, or create an Image element dynamically in your JavaScript code:

var img = new Image();
img.src = "http://somewhere/to/my/image.jpg";

Next, once the image is loaded, draw the image onto a canvas element:

var img = new Image();
img.onload = function() {
    // note: we're calling against the "2d" context here
    mycontext.drawImage(img, 0, 0); // draw the image at (0,0)
};
img.src = "http://somewhere/to/my/image.jpg";