HTML5 Canvas: Save drawing as an image in ASP.Net

HTML

<div class="tools">
    <a href="#colors_sketch" data-tool="marker">Marker</a> <a href="#colors_sketch" data-tool="eraser">
        Eraser</a>
</div>
<br />
<canvas id="colors_sketch" width="500" height="200">
</canvas><br /><br />
<input type="hidden" id="sketch_data" name="sketch_data" />
<asp:Button Text="Upload" runat="server" OnClientClick="SaveSketch()" OnClick="Upload" />
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script src="http://intridea.github.io/sketch.js/lib/sketch.min.js" type="text/javascript"></script>
<script type="text/javascript">
    $(function () {
        $('#colors_sketch').sketch();
        $(".tools a").eq(0).attr("style", "color:#000");
        $(".tools a").click(function () {
            $(".tools a").removeAttr("style");
            $(this).attr("style", "color:#000");
        });
    });
    function SaveSketch(){
        var dataUri = $("#colors_sketch")[0].toDataURL();
        $("#sketch_data").val(dataUri);
    };
</script>

Code

protected void Upload(object sender, EventArgs e)
{
    string sketchData = Request.Form["sketch_data"];
    byte[] bytes = Convert.FromBase64String(sketchData.Split(',')[1]);
    using (var imageFile = new FileStream(Server.MapPath("~/images/Sketch.png"), FileMode.Create))
    {
        imageFile.Write(bytes, 0, bytes.Length);
        imageFile.Flush();
    }
}