Watermarking Images: preserve quality, control opacity

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I set myself a task to create a simple OpenCV program to watermark image. The program will overlay a logo on the bottom right of an image. It will place the logo there with full opacity or slightly transparent. I have some questions whether I have done things correctly and what I can improve.

• Q1: How can I control the opacity of the logo when I call cv::addWeighted()? I have tried different beta values but they always turn out the same more or less
• Q2: Does my code preserve the image quality? I don't think it does, for example; the image I load in is 2mb and when I watermark then save it as a JPEG it is 100kb so I've lost quality. How can I preserve quality?

• Q3: The logo image will always be a PNG with transparent (black?) background. When I threshold to create a mask of the logo I get artifacts or an unclean extraction of the logo. How could I accurately extract the logo without artifacts or losing detail?

  Mat src = imread("./images/pan1.jpg");  
  Mat logo = imread("./images/logo.png"); 
  
  // Watermark image: overlay logo slightly faded/transparent
  Mat watermarked;
  Mat logoResized = Mat(src.rows, src.cols, CV_8UC3);
  // Make the logo image same size as the src image
  copyMakeBorder(logo, logoResized, src.rows - logo.rows, 0, src.cols - logo.cols, 0, BORDER_CONSTANT, { 0,0,0 });
  addWeighted(src, 1.0, logoResized, 1.0, 0, watermarked); // Q1
  imshow("Watermarked Opacity", watermarked);
  imwrite("./images/pan-watermarked1.jpg", watermarked);
  
  
  // Overlay logo
  Mat logoGray, thresh;
  cvtColor(logo, logoGray, CV_BGR2GRAY);
  threshold(logoGray, thresh, 1, 255, CV_THRESH_BINARY);
  Rect r(res.cols-logo.cols, res.rows-logo.rows, logo.cols, logo.rows);
  logo.copyTo(src(r), thresh);
  imshow("Watermarked", src);
  imwrite("./images/pan-watermarked2.jpg", src);