How can I crop an image in Qt?

You can use QPixmap::copy:

QRect rect(10, 20, 30, 40);
QPixmap original('image.png');
QPixmap cropped = original.copy(rect);

There is also QImage::copy:

QRect rect(10, 20, 30, 40);
QImage original('image.png');
QImage cropped = original.copy(rect);