1. Why do you use both QImage and QPixmap?
    Conversion between QImage and QPixmap costs, especially on X Windows, so better use only QPixmaps (unless you have to access individual pixels).
  2. Have you read QPixmap::scaledToWidth() docs?
    If yes, do it again, but this time very carefully. Pay attention especially to the "const" keyword at the end of signature and the return type.