I wrote a sample application, which extract all images from an ICO file.
It uses QImageReader, which uses QtIcoHandler internally.

Expected to see:
All icons should be loaded correctly, including Vista style 256x256 icons.

Got instead:
1) QtIcoHandler does not support Vista style icons stored in PNG (instead of the traditional ICONIMAGE structure with a BITMAPINFOHEADER).
2) QtIcoHandler shrinks icons larger than 128x128, although the highest possible icon size supported should be 256x256
3) ICOReader::write() should write 256x256 icons in PNG format into the ICO file.
4) It is not possible to create an icon, with more images