What is the data in aegea.png, because it is this you are manipulating (you discard the unitialised Format_ARGB32 QImage)? Are you doing anything with the QImage other than loading the file? If not, then why not just load the QPixmap directly?

My example image is identified (ImageMagick) as:
Qt Code: 
  1. $ identify -verbose plane.png
  2. Image: plane.png
  3.   Format: PNG (Portable Network Graphics)
  4.   Class: DirectClass
  5.   Geometry: 1040x138+0+0
  6.   Resolution: 72x72
  7.   Print size: 14.4444x1.91667
  8.   Units: Undefined
  9.   Type: TrueColorMatte
  10.   Endianess: Undefined
  11.   Colorspace: RGB
  12.   Depth: 8-bit
  13.   Channel depth:
  14.     red: 8-bit
  15.     green: 8-bit
  16.     blue: 8-bit
  17.     alpha: 8-bit
  18. ...
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and is a mostly transparent banner of an aircraft with partial transparency of the spinning propeller. This code:
Qt Code: 
  1. // Your code with adjustment for static method
  2.     QImage overlay_image(768,768,QImage::Format_ARGB32); 
  3.     bool loaded = overlay_image.load ("plane.png",  0 ); 
  4.     QPixmap overlay_map = QPixmap::fromImage ( overlay_image,Qt::AutoColor );
  5.     qDebug() << loaded;
  6.     qDebug() << overlay_map.size() << overlay_map.depth() << overlay_map.hasAlphaChannel();
  7.  
  8. // Load the PNG directly
  9.     overlay_map.load("plane.png");
  10.     qDebug() << overlay_map.size() << overlay_map.depth() << overlay_map.hasAlphaChannel();
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outputs
Qt Code: 
  1. true 
  2. QSize(1040, 138) 32 true 
  3. QSize(1040, 138) 32 true
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on Linux, Qt 4.7.2.