Bitmapをただ保存して読み込むだけでdpiが変わってしまうのですが、
ライブラリのバグですか？　pngとbmpでも結果が違うし

private void Form1_Load(object sender, EventArgs e)
{
Bitmap bmp1 = new Bitmap(16, 16);
bmp1.Save("test.png");
bmp1.Save("test.bmp", System.Drawing.Imaging.ImageFormat.Bmp);
Trace.WriteLine(string.Format("H:{0}, V:{1}", bmp1.HorizontalResolution, bmp1.VerticalResolution));

Bitmap bmp2 = new Bitmap("test.png");
Trace.WriteLine(string.Format("H:{0}, V:{1}", bmp2.HorizontalResolution, bmp2.VerticalResolution));

Bitmap bmp3 = new Bitmap("test.bmp");
Trace.WriteLine(string.Format("H:{0}, V:{1}", bmp3.HorizontalResolution, bmp3.VerticalResolution));
}

H:96, V:96
H:95.9866, V:95.9866
H:96.01199, V:96.01199