transparent GIF pixels

[Kenneth Mankoff <mankoff(at)lasp.colorado.edu>]

Hi all,
  
Is it possible to create transparent pixels? 

It appears from the help that you can, but only when you write png's to
disk. I have tried this and have been unsuccessful.

Anyways, i want to do it in a situation where i don't write and immediatly
read just to get my transparencies. I would like to make them transparent
while the image is in memory. And i would prefer gifs, not png's.

thanks for any clues or pointers.

-ken.