Here's what I do for "keeping" areas of images transparent:

- Import my GIF into a graphic symbol (JPG should work too, but I've not tried it)
- Modify > Break Apart the image
- Deselect everything
- Now, using the Magic Wand from the Lasso tool I select the solid areas and then just press delete. I use Flash 3 and the Lasso is found in the main tools palette. When I choose this tool, more tools appear below, one of which is the Magic Wand.

See if this works

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