Make a useful function like this:
IplImage* img_resize(IplImage* src_img, int new_width,int new_height)
{
IplImage* des_img;
des_img=cvCreateImage(cvSize(new_width,new_height),src_img->depth,src_img->nChannels);
cvResize(src_img,des_img,CV_INTER_LINEAR);
return des_img;
}