Setting certain colours to transparent

When I first started making websites on my Mac 4 years ago, one of the things that you could do to imported gifs was to set a certain colour to be transparent. This ment that if you wanted to insert a picture of a donought you would drar a poicture of a donought with a pink backgound. When you inserted the donought into the webpage you selected the gif and set all instances of the colour pink to transparant.

I have several flash movies (animations) that involve round and hollow objects. When I draw them in photoshop I have a transparent background but when I copy them to Flash, all the transparent sections turn white. Is it possible to turn those section of the pictures back to transparent and would I have to do it to each individual keyframe or is there some way I can just say that everything that is white becomes transparent.

Cheers,
Carl