效果杠杠的！！

//图像差分
#include <stdio.h>
#include <stdlib.h>
#include "cv.h"
#include "highgui.h"
void Image_Minus(IplImage *X, IplImage *Y, IplImage *X_Y)
{
	//图像差分函数，将图像1中像素和图像2中对应像素想减，要求X、Y、X_Y大小相同
	int i,j,width,height,step,chanel;
	unsigned char *dataX, *dataY, *dataX_Y;
	width = X->width;
	height = X->height;
	dataX = (unsigned char *)X->imageData;
	dataY = (unsigned char *)Y->imageData;
	dataX_Y = (unsigned char *)X_Y->imageData;
	step = X->widthStep/sizeof(char);
	chanel = X->nChannels;
	for(i=0; i<height; i++)
		for(j=0; j<width*chanel; j++)
			dataX_Y[i*step+j] = abs( dataX[i*step+j] - dataY[i*step+j]);
}
int main()
{
	IplImage* pImgX;
	IplImage* pImgY;
	IplImage* pImgX_Y;
	CvSize dest_size;
	pImgX = cvLoadImage("D://bishe//07_31_10_09_41_08.bmp", -1);
	pImgY = cvLoadImage("D://bishe//07_31_10_11_40_48.bmp", -1);
	
	if(pImgX==0 || pImgY==0)
	{
		printf("载入文件失败！/n");
		return -1;
	}
	dest_size.width = pImgX->width;
	dest_size.height = pImgX->height;
	pImgX_Y = cvCreateImage(dest_size, pImgX->depth, pImgX->nChannels);
	//图像差分
	Image_Minus(pImgX, pImgY, pImgX_Y);
	//创建窗口
	cvNamedWindow("Picture X:", 1);
	cvNamedWindow("Picture Y:", 1);
	cvNamedWindow("Picture X-Y:", 1);
	//显示图像
	cvShowImage("Picture X:", pImgX);
	cvShowImage("Picture Y:", pImgY);
	cvShowImage("Picture X-Y:", pImgX_Y);
	cvWaitKey(0);
	//销毁窗口
	cvDestroyWindow("Picture X:");
	cvDestroyWindow("Picture Y:");
	cvDestroyWindow("Picture X-Y:");
	//释放图像
	cvReleaseImage(&pImgX);
	cvReleaseImage(&pImgY);
	cvReleaseImage(&pImgX_Y);
	return 0;
}

加注释版：

//图像差分
#include <stdio.h>
#include <stdlib.h>
#include "cv.h"
#include "highgui.h"
#include <iostream>

using namespace std;

void Image_Minus(IplImage *X, IplImage *Y, IplImage *X_Y)
{
	//图像差分函数，将图像1中像素和图像2中对应像素想减，要求X、Y、X_Y大小相同
	int i,j,width,height,step,chanel;
	unsigned char *dataX, *dataY, *dataX_Y;

	width = X->width;
	height = X->height;

	//存入矩阵数据
	dataX = (unsigned char *)X->imageData;
	dataY = (unsigned char *)Y->imageData;
	dataX_Y = (unsigned char *)X_Y->imageData;

	//计算步长
	step = X->widthStep/sizeof(char);
	chanel = X->nChannels;

	//一个个数据处理
	for(i=0; i<height; i++)
		for(j=0; j<width*chanel; j++)
			dataX_Y[i*step+j] = abs( dataX[i*step+j] - dataY[i*step+j]);
}


int main()
{
	IplImage* pImgX;
	IplImage* pImgY;
	IplImage* pImgX_Y;
	CvSize dest_size;
	pImgX = cvLoadImage("mmr_out.jpg", -1);
	pImgY = cvLoadImage("zcr_out.jpg", -1);

	if(pImgX==0 || pImgY==0)
	{
		printf("载入文件失败！/n");
		return -1;
	}
	dest_size.width = pImgX->width;
	dest_size.height = pImgX->height;

	cout<<"width == "<<dest_size.width<<endl;
	cout<<"height == "<<dest_size.height<<endl;

	pImgX_Y = cvCreateImage(dest_size, pImgX->depth, pImgX->nChannels);
	
	//图像差分，最最关键的一步
	Image_Minus(pImgX, pImgY, pImgX_Y);


	//创建窗口
	cvNamedWindow("Picture X:", 1);
	cvNamedWindow("Picture Y:", 1);
	cvNamedWindow("Picture X-Y:", 1);
	//显示图像
	cvShowImage("Picture X:", pImgX);
	cvShowImage("Picture Y:", pImgY);
	cvShowImage("Picture X-Y:", pImgX_Y);
	cvWaitKey(0);
	//销毁窗口
	cvDestroyWindow("Picture X:");
	cvDestroyWindow("Picture Y:");
	cvDestroyWindow("Picture X-Y:");
	//释放图像
	cvReleaseImage(&pImgX);
	cvReleaseImage(&pImgY);
	cvReleaseImage(&pImgX_Y);
	return 0;
}

OpenCv图像差分