Hi zogjohnny,

Please refer below Sample.

HTML

<asp:ScriptManager runat="server" />
<asp:UpdatePanel runat="server" ID="updTools">
    <ContentTemplate>
        <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 />
        <asp:Button Text="Save as Image" runat="server" ID="btnSave" />
        <hr />
        <img id="imgCapture" alt="" />
    </ContentTemplate>
</asp:UpdatePanel>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript" src="https://cdn.rawgit.com/mobomo/sketch.js/master/lib/sketch.min.js"></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");
        });
        $("#btnSave").bind("click", function () {
            var base64 = $('#colors_sketch')[0].toDataURL();
            $("#imgCapture").attr("src", base64);
            $("#imgCapture").show();
            $.ajax({
                type: 'POST',
                url: 'Default.aspx/SaveImage',
                data: '{ "imageData" : "' + base64 + '" }',
                contentType: 'application/json; charset=utf-8',
                dataType: 'json',
                success: function (response) {
                    alert('Image saved successfully !');
                }
            });
            return false;
        });
    });
</script>

Namespaces

C#

using System.Web.Services;
using System.IO;

VB.Net

Imports System.Web.Services
Imports System.IO

Code

C#

[WebMethod]
public static void SaveImage(string imageData)
{
    string filePath = HttpContext.Current.Server.MapPath("~/HTML.png");
    using (FileStream fs = new FileStream(filePath, FileMode.Create))
    {
        using (BinaryWriter bw = new BinaryWriter(fs))
        {
            byte[] data = Convert.FromBase64String(imageData.Split(',')[1]);
            bw.Write(data);
            bw.Close();
        }
    }
}

VB.Net

<WebMethod>
Public Shared Sub SaveImage(ByVal imageData As String)
    Dim filePath As String = HttpContext.Current.Server.MapPath("~/HTML.png")
    Using fs As FileStream = New FileStream(filePath, FileMode.Create)
        Using bw As BinaryWriter = New BinaryWriter(fs)
            Dim data As Byte() = Convert.FromBase64String(imageData.Split(","c)(1))
            bw.Write(data)
            bw.Close()
        End Using
    End Using
End Sub

Screenshot