Using a bitmap, the two colors can be any two colors, not just black and white.
I saved a PICT as 32-bit with no compression--no success. Saving the same PICT as 16-bit works. However, I needed one solid color to pick for the transparency.
I worked with the 16-bit PICT, adding an alpha channel. None of the options in the graphics mode recognized the alpha channel. That might be because I copied it from PictureViewer.