Opengl and PGM display

[OneWayOrAnother]

Hello guys.
I want to show some pgm image using OpenGL.
I achieved that pretty easy with this:

void OGLPyramid::display(const QImage* img)
{
    m_image = img;
    this->update();


}

void OGLPyramid::paintEvent(QPaintEvent *)
{
    QPainter painter(this);
    painter.drawImage(this->rect(), *m_image);

}

But i got a question - how can i specify display range? like if image got brightness distribution is between 0 and 65535 it looks pretty much okay, but if distribution is between 5000 and 5500 it looks black since display range stays from 0 to 65535. How can i adjust display range? Ofc would be nice to avoid changing the initial img data.

Thanks for your help.