logo.copyTo(imageROI,mask)

using namespace cv;

int main(int argc, char** argv)

{

 Mat img1 = imread("lena.jpg");

 Mat logo = imread("logo.png");

 if(img1.empty() || logo.empty())

 {

  printf("Can't read one of the images\n");

  return -1;

 }

 Mat imageROI=img1(cv::Rect(img1.cols-logo.cols,img1.rows-logo.rows,

  logo.cols,logo.rows));

 cv::Mat mask(imread("logo.png", CV_LOAD_IMAGE_GRAYSCALE));

 logo.copyTo(imageROI,mask);

 cv::imshow("image",img1);

 cv::waitKey(0);

 return 0;

}