Put a button on the qlabel that fills the screen

MyNameIsQt

Fill the window with an image like the one pictured above.
And the button is located in the middle below.
But my code doesn't work like that. The imageLabel is just very small and centered.
where do i need to fix it?

ui->setupUi(this);
    mainLayout = new QVBoxLayout;
    QLabel *imageLabel = ui->label;

    imageLabel->setStyleSheet("QLabel { background-color : yellow; color : blue; }");
    imageLabel->setBackgroundRole(QPalette::Base);
    imageLabel->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
    imageLabel->setScaledContents(true);
    mainLayout->addWidget(imageLabel, 0, Qt::AlignCenter);

    buttonLayout = new QHBoxLayout;
    buttonLayout->addWidget(ui->beginBtn);
    buttonLayout->addWidget(ui->prevBtn);
    buttonLayout->addWidget(ui->NextBtn);
    buttonLayout->addWidget(ui->endBtn);
    buttonLayout->setAlignment(Qt::AlignBottom | Qt::AlignHCenter);

    mainLayout->addLayout(buttonLayout);
    setCentralWidget(new QWidget);
    centralWidget()->setLayout(mainLayout);

    createStatusBar();

If I do this part as follows, the qlabel is filled, but the button is pushed below it.

mainLayout->addWidget(imageLabel);

CPPUIX

If you have to use a QLabel for your background (because there better ways to do so) then I suggest using its geometry.
set its pos to (0,0), and size to the size of your window or central widget, and you might also need to change its Z-Order in which case you can use lower or raise functions
this topic might be of use : Bring widget at the back of another widget

JoeCFD

@MyNameIsQt If I understand it correctly, you want to add two buttons on top of label image. Then

buttonLayout = new QHBoxLayout( imageLabel ); // this layout is for imageLabel

comment this out
mainLayout->addLayout(buttonLayout);
add
mainLayout->addWidget( imageLabel );

Pl45m4

@MyNameIsQt

I dont know the purpose of your image, but you could also fill the background of your centralWidget with it (by using stylesheet, for example) and then simply add your buttons to it.

CPPUIX

If you have to use a QLabel for your background (because there better ways to do so) then I suggest using its geometry.
set its pos to (0,0), and size to the size of your window or central widget, and you might also need to change its Z-Order in which case you can use lower or raise functions
this topic might be of use : Bring widget at the back of another widget