C++ (Qt)
class MyButton : public QPushButton
{
    Q_OBJECT
public:
    MyButton(QWidget * parent = 0) : QPushButton(parent) {
        pixmap.load(QLatin1String("../../qtcreator_logo_256.png"));
        setFixedSize(pixmap.size());
        setMask(pixmap.mask());
    }
protected:
    void paintEvent(QPaintEvent * event) {
        QPainter painter(this);
        painter.drawPixmap(rect(), pixmap);
    }
private:
    QPixmap pixmap;
};