How do I add a background image to the QMainWindow?

You can add a background image to your MainWindow by doing the following:

1. create a QPixmap and give it the path to your image.
2. create a QPalette and set it’s QBrush with your pixmap and it’s ColorRole to QPalette::Background.
3. set your MainWindow palette to the palette you created.

as an example you can add this lines to the constructor of your MainWindow class:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QPixmap bkgnd("/home/user/Pictures/background.png");
    bkgnd = bkgnd.scaled(this->size(), Qt::IgnoreAspectRatio);
    QPalette palette;
    palette.setBrush(QPalette::Background, bkgnd);
    this->setPalette(palette);
}

the advantage with this one is that you have the ability of modifying/changing your background image programmatically without having to use or learn any css stylesheet syntaxes.