如何用c＃读取gif和jpg的图象象素信息

给你一段我编的代码，功能是变图片的分辨率，一看就明白了
Color c = new Color();
Bitmap box1 = new Bitmap(pictureBox1.Image);
Bitmap box2 = new Bitmap(pictureBox2.Image);
int r, g, b, i, j, size, k1, k2, xres, yres;
xres = pictureBox1.Image.Width;
yres = pictureBox1.Image.Height;
size = 2;
for (i = 0; i < (xres - 1);i+=size)
{
for (j = 0; j <( yres - 1); j += size)
{
c = box1.GetPixel(i,j);//取像素点
r = c.R;
g = c.G;
b = c.B;

Color cc = Color.FromArgb(r,g,b);
for (k1 = 0; k1 <= (size - 1);k1++)
{
for (k2 = 0; k2 <= (size - 1); k2++)
{
box