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closing and re-opening a capture is usually a bad idea (slow/memory intensive), so i'd rather change the logic / code-flow in your program: have 1 button to take a picture, and another to quit.

VideoCapture cap;
if (!cap.open(0)) {
    return 0;}
for (;;)
{
    cap >> frame;
    if (frame.empty()) break; // end of video stream
    imshow("Smile! Press space to take a picture, esc to stop.", frame);
    int k = waitKey(1);
    if (k == ' ') // space
    {
        imwrite("../image.png", frame);
    }
    if (k == 27) // escape
    {
        break; // stop capturing by pressing ESC 
    }
}
// the camera will be closed automatically upon exit

closing and re-opening a capture is usually a bad idea (slow/memory intensive), so i'd rather change the logic / code-flow in your program: have 1 button to take a picture, and another to quit.

int imagesTaken=0;
VideoCapture cap;
if (!cap.open(0)) {
    return 0;}
for (;;)
{
    cap >> frame;
    if (frame.empty()) break; // end of video stream
    imshow("Smile! Press space to take a picture, esc to stop.", frame);
    int k = waitKey(1);
    if (k == ' ') // space
    {
        imwrite("../image.png", frame);
imwrite(format("../image%04d.png", imagesTaken), frame); // try unique name
        imagesTaken++;
    }
    if (k == 27) // escape
    {
        break; // stop capturing by pressing ESC 
    }
}
// the camera will be closed automatically upon exit