string destination = "/sdcard/DCIM/";
    //判断目录是否存在，不存在则会创建目录
    if (!Directory.Exists(destination))
    {
        Directory.CreateDirectory(destination);
    }
    string path = destination + "/" + "xxylShare.PNG";
    //存图片
    System.IO.File.WriteAllBytes(path, shareTex);

发布前要设置cd权限，在playersetting/othersetting下，如图