You have to pass Qt::SmoothTransformation
transformation mode to the scaled
function like:
QImage img(name);
QPixmap pixmap;
pixmap = pixmap.fromImage(img.scaled(width,height,Qt::IgnoreAspectRatio,Qt::SmoothTransformation));
QFile file(folder+"/"+name);
file.open(QIODevice::WriteOnly);
pixmap.save(&file, "jpeg",100);
file.close();