Overlaying an image with transparency?

[Ronald Kneusel <rtk cmed com>]

I have a drawing area with an image in it and want to overlay another
image.  The second image is binary, 1 means that pixel should be drawn
and 0 means that the pixel is to be transparent.

Is there a way to overlay this image without actually moving through it
and either drawing the pixel or not based on the value?  Can I make a
type of colormap with 2 colors only, one transparent?

Any help appreciated!

Ron Kneusel
rtk cmed com