Couple things:
Use drawPixmap. QImage is optimized for editing, and QPixmap is optimized for display. So if you are painting often, then do it with QPixmap and drawPixmap (drawImage will internally convert to a QPixmap which is slow).

Your drawing code seems alright. You could try setting the SmoothPixmapTransform render hint on the view, although that shouldn't make a difference.

Are you drawing your image in full and unscaled, scaled, or clipped? Because if you're scaling your image and are using something like QPixmap::scaled(), then you have to pass it the Qt::SmoothTransformation transformation mode. Post the code where you load your pixmap/image.