Printer transparency on Win32

I’m trying to do transparent printing on Win32 and I get PlatformNotSupported error when trying to set the printer’s graphics object transparency property. I read the docs and it states that transparency only works on Picture with a depth of 32, or is a Picture with an alpha channel. Understood, but there seems to be no way to get the OpenPrinterDialog to return a graphics object that meet this criteria. How doI do this?