It just takes a setting in the manifest file.
Based on my experiences with the new 256MB emulator (which is included with the 7.1.1 SDK update) the new, lower spec devices don't appear to support the 32bit option. This would make sense as it would mean the device would need to use less memory to load/display the image and could also mean a cheaper screen.
This is currently only tested in the emulator but as soon as I can get a real device I'll check there too.
Do you have a device you can check this on? If so, do you see this behaviour?
Need to do some dithering of images to wrok around this? Check out http://adtsai.blogspot.co.uk/2011/03/high-quality-dithering-for-windows.html