//QComboBox *linestyleBox;

linestyleBox->setIconSize(QSize(40, 10));
QPen pen;
QPainter paint;
pen.setWidth(3);
pen.setColor(QColor(Qt::black));
paint.begin(&pixmap);

//Solid
pixmap.fill(QColor(Qt::white));
pen.setStyle(Qt::SolidLine);
paint.setPen(pen);
paint.drawLine(0, 5, 40, 5);
linestyleBox->insertItem(0, QIcon(pixmap), "Solid line", Qt::SolidLine);
//DashLine
pixmap.fill(QColor(Qt::white));
pen.setStyle(Qt::DashLine);
paint.setPen(pen);
paint.drawLine(0, 5, 40, 5);
linestyleBox->insertItem(0, QIcon(pixmap), "Dash line", Qt::DashLine);
//... другие типы линий