If the only thing you want to do is drop in an image onto a widget withouth the complexity of the graphics API, you can also just create a new QWidget and set the background with StyleSheets. Something like this:
MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
...
QWidget *pic = new QWidget(this);
pic->setStyleSheet("background-image: url(test.png)");
pic->setGeometry(QRect(50,50,128,128));
...
}