How to draw on zoomed image in Qt - c++

Question

I'm working on a project where I have to do an image editor similar to Paint. I would like to open up an image, zoom in/zoom out if necessary, and then draw on it.
For that, I used Image Viewer and Scribble examples, the only thing different that I added was a QLabel subclass that is supposed to draw lines and other forms with the mouse press/mouse release events. The problem is with the overriden paintEvent.
void imLabel::paintEvent(QPaintEvent *event){
if(tipo != ""){ //draw mode
QPainter painter(this);
QRect dirtyRect = event->rect();
painter.drawImage(dirtyRect,image,dirtyRect);
}
else QLabel::paintEvent(event);
}
I am able to zoom in normally in the beginning, but when I start drawing the image comes back to its original size, while the rest of the label that is not occupied by the image is completely white, because it kept the zoomed in size.
Is there a way that I can keep the image zoomed in while I draw, just like in Paint?
UPDATE
Here's the code of the draw function, maybe it'll help
void imLabel::draw(const QPoint &endPoint){
QPainter painter(&image);
painter.setPen(QPen(myPenColor, myPenWidth, Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin));
if(tipo == "maoLivre" || tipo == "reta") //draw line or free hand
painter.drawLine(startPoint, endPoint);
else{
int x = startPoint.x(), y = startPoint.y(),
largura = endPoint.x()- startPoint.x(),
altura = endPoint.y() - startPoint.y();
if(tipo == "retangulo") //draw rectangle
painter.fillRect(x,y,largura,altura,Qt::green);
else if(tipo == "circulo"){ //draw circle
painter.setBrush(Qt::green);
painter.drawEllipse(startPoint,altura,altura);
}
}
modified = true;
update();
startPoint = endPoint;
}