You cannot use the image object directly, you need to convert the image to canvas, then convert the canvas todataUrl.
var image = new Image(50, 50);
image.src = args.cellValue;
var canvas = document.createElement("canvas");
canvas.width = image.width;
canvas.height = image.height;
canvas.getContext("2d").drawImage(image, 0, 0);
var imageUrl = canvas.toDataURL();
var imageShape = new $.ig.excel.WorksheetImage(imageUrl );