The PictureBox.SizeMode options are missing a "fill" or "cover" mode which would be like zoom except with cropping to ensure you're filling the picture box. In CSS it's the "cover" option.
This code should enable that:
static public void fillPictureBox(PictureBox pbox, Bitmap bmp)
{
pbox.SizeMode = PictureBoxSizeMode.Normal;
bool source_is_wider = (float)bmp.Width / bmp.Height > (float)pbox.Width / pbox.Height;
var resized = new Bitmap(pbox.Width, pbox.Height);
var g = Graphics.FromImage(resized);
var dest_rect = new Rectangle(0, 0, pbox.Width, pbox.Height);
Rectangle src_rect;
if (source_is_wider)
{
float size_ratio = (float)pbox.Height / bmp.Height;
int sample_width = (int)(pbox.Width / size_ratio);
src_rect = new Rectangle((bmp.Width - sample_width) / 2, 0, sample_width, bmp.Height);
}
else
{
float size_ratio = (float)pbox.Width / bmp.Width;
int sample_height = (int)(pbox.Height / size_ratio);
src_rect = new Rectangle(0, (bmp.Height - sample_height) / 2, bmp.Width, sample_height);
}
g.DrawImage(bmp, dest_rect, src_rect, GraphicsUnit.Pixel);
g.Dispose();
pbox.Image = resized;
}