//签字图片Base64格式去除开头多余字符data:image/png;base64,
strSignImg = strSignImg.Substring(strSignImg.IndexOf(‘,‘) + 1);
//签名Base64生成图片，返回图片路径
string rootPath = AppContext.BaseDirectory;
//图片生成路径
string signImgPath = Path.Combine(rootPath, "UploadFiles\\SignImgs");
//如果不存在就创建 SignImgs 文件夹  
if (!Directory.Exists(signImgPath))
{
    Directory.CreateDirectory(signImgPath);
}
//输出图片文件
File.WriteAllBytes(signImgPath + "\\SignImg_" + DateTime.Now.Ticks.ToString() + ".png", Convert.FromBase64String(strSignImg));//将base64转成图片