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That is because of two things 1) You are using the old C - syle API 2) Once a namedWindow has been made, the container keeps existing, however, you should give it the parameter autosize.

What I suggest is using the newer C++ API and the following code

Mat img = imread("jpgTest.jpg"); 

// show source image
namedWindow("Image", WINDOW_AUTOSIZE); 
imshow("Image",img);

// wait on keystroke to close down
waitKey(0);