Image resizing hangs

const QFuture<QImage> resized_images = QtConcurrent::mapped(image_paths, Picture_load_scale_multithread(m_picture_size_spinbox->value()));
while(!resized_images.isFinished()){...}

struct Picture_load_scale_multithread{
// constructor:
Picture_load_scale_multithread(const int max_size) : m_max_width(max_size), m_max_height(max_size) {} // define output type typedef QImage result_type; // member:
const int m_max_width;
const int m_max_height;

QImage operator()(const QString& image_path){
if(image_path.isEmpty()){
return QImage();
}

QImageReader imageReader(image_path);
QSize s = imageReader.size();
// s = QSize(); // UNCOMMENT TO FORCE USAGE OF QImage::scale INSTEAD OF QImageReader.setScaledSize

if(s.isValid() && !s.isNull()){// use of imageReader scaling should be faster
// calculate QSize to scale to, to keep Image aspect ratio (Qt::KeepAspectRatio is not a direct option with QImageReader)

const qreal f = qMin(static_cast<qreal>(m_max_width)/static_cast<qreal>(s.width()), static_cast<qreal>(m_max_height)/static_cast<qreal>(s.height())); imageReader.setScaledSize(QSize(qCeil(f*static_cast<qreal>(s.width())), qCeil(f*static_cast<qreal>(s.height()))));

const QImage i_scaled = imageReader.read(); // LARGE IMAGE HANGS HERE

if(i_scaled.isNull()){
Pf::debug_other_log("Picture_load_scale_multithread", QObject::tr("Output of scaling picture is NULL picture, error is:\n")+imageReader.errorString());
}
return i_scaled;
}else{ 
//use QImage::scale

Pf::debug_other_log("Picture_load_scale_multithread", "cannot get QSize for image : "+image_path);
const QImage image = imageReader.read();
if(image.isNull()){
Pf::debug_other_log("Picture_load_scale_multithread", "Null image : "+image_path);
return image;
}else if(m_max_width >= image.width() && m_max_height >= image.height()){
//Avoid scaling to bigger image
return image;
}else{
const QImage i_scaled = image.scaled(m_max_width, m_max_height, Qt::KeepAspectRatio, Qt::SmoothTransformation); // SOME IMAGES SCALING HANGS

if(i_scaled.isNull()){
Pf::debug_other_log("Picture_load_scale_multithread", QObject::tr("Output of scaling picture is NULL picture"));
} 

return i_scaled;
}
}
//return QImage(); never reached
}
};