preventing GDI+ from scaling images

Hi,

I am loading PNG files with the Gdiplus::Bitmap() constructor. To display 
them I create a CompatibleBitmap(). In case the resolution(DPI) of the 
bitmap/PNG does not match the resolution (DPI) of the screen context 
(accessed via Gdiplus::Graphics) the bitmaps are draw scaled. It only works 
"pixelwise" when the resolutions match.

I tried to call bitmap.SetResolution() but afterwards the 
Gdiplus::Graphics::DrawImage() function returns "InvalidParameter".

How can I make GDI+ ignoring the DPI within the file and just use pixels?

thanks for any help

docschnipp

I'd like to add that I call this DrawImage() method:
  DrawImage(Image, dx,dy, sx,sy,sw,sh, UnitPixel);

which is the only suitable option in this case, since I need to draw only 
portions of the bitmap.

thanks
docschnipp