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;
}