Using implicit sharing to return an image as a method parameter

[Luc4]

Hi! I've been stuck with this for hours, I don't know why I'm doing this wrong, can't understand.
What I want to do is to create a method that returns a QImage, and pass this QImage not a the return type, but as a parameter of the method (I know I can do it other ways, but I really would like to learn why this is wrong and how it should be done).
This is more or less what I did:

QImage myImage;
myMethod(myImage);

QImage myImage;
myMethod(myImage);

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where myMethod is defined as:

bool MyClass::myMethod(QImage& myImage)
{
   ...
   myImage = QImage::fromData(...);
   ...
   return true;
}

bool MyClass::myMethod(QImage& myImage)
{
   ...
   myImage = QImage::fromData(...);
   ...
   return true;
}

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I thought that implicit sharing used this way should have assigned the new image to my myImage which, being passed by reference, is my original image. Unfortunately this is wrong as I can see. Why?

Thanks!