draw with QPaint on loaded QPixmap does not work

Question

I can draw a Line on my Qlabel by doing this:

    QPixmap *myPixmap;
    myPixmap = new QPixmap();
    myPixmap->fill(Qt::red);
    QPainter painter(myPixmap);
    painter.setPen(Qt::blue);
    painter.drawLine(1, 1, 50, 50);
    ui.label->setPixmap(*myPixmap);

That works. I get a red label with a blue line.

But now i want do draw that line over a 1.jpg like this:

    QPixmap *myPixmap;
    myPixmap = new QPixmap(ui.label->size());
    myPixmap->load("c:\\temp\\1.jpg");
    QPainter painter(myPixmap);
    painter.setPen(Qt::blue);
    painter.drawLine(1, 1, 50, 50);
    ui.label->setPixmap(*myPixmap);

Unfortunately now i only get my 1.jpg but no blue line over it.

Can anybody explain this? I work on Win10 / QT5.6.1

---

Edit to clarify my mistake.

Thanks to the comments of @musicamante and @A.R.M i figured out that the blue line was there but just out of sight because it was to large for the Mainwindow. Unfortunately my original 1.jpg was unicolored so i did not notice that.

I added a frame to my 1.jpg so i can now show the problem i had with 2 screenshots:

with line without line