public static string ImgToBase64String(Bitmap bmp)
        {
            try
            {
                MemoryStream ms = new MemoryStream();
                bmp.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
                byte[] arr = new byte[ms.Length];
                ms.Position = 0;
                ms.Read(arr, 0, (int)ms.Length);
                ms.Close();
                return Convert.ToBase64String(arr);
            }
            catch (Exception ex)
            {
                return null;
            }
        }

data:image/png;base64,

Bitmap bt;
            string base64Img = ImgToBase64String(bt);
            this.Image1.ImageUrl = "data:image/png;base64," + base64Img;