Converting image file format

[Valentinor]

I am trying to make an add-on for Mozilla Firefox that will convert an image (from a given direct link) to another format and then give the user the save file pop-up. I have tried to use JavaScript to turn the image into a canvas, and then the canvas to an image with the format wanted, but I run into the problem with canvas.toDataURL, that doesn't allow to use an image from another origin, and I get Uncaught DOMException: The operation is insecure. How else can I do this image conversion? I know it is possible to do it, because there are other add-ons that do this, but my add-on will have other specific functions for our work beside only image conversion, and we can't use those because of that. I'm doing the testing in straight HTML, because it is easier to do it this way, then keep refreshing the add-on in browser. Here is the code:

window.onload = function() {
var image = new Image();
image.addEventListener("load", imageLoaded, false);
image.src = "theLinkToAnImage";
};

function imageLoaded() {
var canvas = document.createElement("canvas");
canvas.width = this.width;
canvas.height = this.height;
canvas.getContext("2d").drawImage(this, 0, 0);

var image = new Image();
image.crossOrigin = "Anonymous";
image.src = canvas.toDataURL("image/jpg");
document.body.appendChild(image);

}