How can I iterate through each pixel in a .gif image?

This is a complete example with both methods, using LockBits() and GetPixel(). Besides the trust issues with LockBits() things can easily get hairy.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Drawing;

namespace BitmapReader
{
    class Program
    {
        static void Main(string[] args)
        {
            //Try a small pic to be able to compare output, 
            //a big one to compare performance
            System.Drawing.Bitmap b = new 
                System.Drawing.Bitmap(@"C:\Users\vinko\Pictures\Dibujo2.jpg"); 
            doSomethingWithBitmapSlow(b);
            doSomethingWithBitmapFast(b);
        }

        public static void doSomethingWithBitmapSlow(System.Drawing.Bitmap bmp)
        {
            for (int x = 0; x < bmp.Width; x++)
            {
                for (int y = 0; y < bmp.Height; y++)
                {
                    Color clr = bmp.GetPixel(x, y);
                    int red = clr.R;
                    int green = clr.G;
                    int blue = clr.B;
                    Console.WriteLine("Slow: " + red + " " 
                                       + green + " " + blue);
                }
            }
        }

        public static void doSomethingWithBitmapFast(System.Drawing.Bitmap bmp)
        {
            Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);

            System.Drawing.Imaging.BitmapData bmpData =
                bmp.LockBits(rect, 
                    System.Drawing.Imaging.ImageLockMode.ReadOnly,
                    bmp.PixelFormat);

            IntPtr ptr = bmpData.Scan0;

            int bytes = bmpData.Stride * bmp.Height;
            byte[] rgbValues = new byte[bytes];

            System.Runtime.InteropServices.Marshal.Copy(ptr, 
                           rgbValues, 0, bytes);

            byte red = 0;
            byte green = 0;
            byte blue = 0;

            for (int x = 0; x < bmp.