C#:尝试向图像添加缩略图符号时结果不同

我在 ASP.NET 应用程序中自动调整图像大小，以便创建该图像的低分辨率缩略图。此代码工作正常。调整大小后，我正在尝试向该图像添加"缩略图符号"，例如小放大镜或加号，但结果因图像大小而异。

请注意：原始图像仅调整为特定宽度，因此图像的高度不同。

我的代码如下所示：

private static byte[] InsertThumbnailSign(byte[] imageBuffer, string signPath)
{
    byte[] output = null;
    MemoryStream stream = new MemoryStream(imageBuffer);
    Image image = Image.FromStream(stream);
    // Add the thumbnail-sign
    Image thumbNailSign = Image.FromFile(signPath);
    Graphics graphic = Graphics.FromImage(image);
    graphic.DrawImageUnscaled(thumbNailSign, image.Width - thumbNailSign.Width - 4, image.Height - thumbNailSign.Height - 4);
    graphic.Flush();
    MemoryStream memoryStream = new MemoryStream();
    image.Save(memoryStream, ImageFormat.Jpeg);
    output = new byte[memoryStream.Length];
    memoryStream.Position = 0;
    memoryStream.Read(output, 0, (int)memoryStream.Length);
    memoryStream.Close();
    stream.Dispose();
    graphic.Dispose();
    memoryStream.Dispose();
    return output;
}

在我看来，缩略图标志应该具有恒定的大小，但事实并非如此。您有什么想法如何实现这一目标吗？

编辑

：刚刚编辑了代码以了解不同的分辨率。但它仍然不起作用：

private static byte[] InsertThumbnailSign(byte[] imageBuffer, string signPath)
{
    byte[] output = null;
    MemoryStream stream = new MemoryStream(imageBuffer);
    Image image = Image.FromStream(stream);
    // Add the thumbnail sign with resolution of the containing image
    Bitmap t = (Bitmap)Bitmap.FromFile(signPath);
    t.SetResolution(image.HorizontalResolution, image.VerticalResolution);
    Image thumbNailSign = t;
    Graphics graphic = Graphics.FromImage(image);
    graphic.DrawImageUnscaled(thumbNailSign, image.Width - thumbNailSign.Width - 4, image.Height - thumbNailSign.Height - 4);
    graphic.Flush();
    MemoryStream memoryStream = new MemoryStream();
    image.Save(memoryStream, ImageFormat.Png);
    output = new byte[memoryStream.Length];
    memoryStream.Position = 0;
    memoryStream.Read(output, 0, (int)memoryStream.Length);
    memoryStream.Close();
    stream.Dispose();
    graphic.Dispose();
    memoryStream.Dispose();
    return output;
}