Wayne,

I made some screen shots to demo the image optimization and gif transparency
settings.

Once you get your button artwork in the preview window, set the export to GIF.
Then click the eye dropper tool at the bottom. See here:
http://joedasilva.com/work/misc/eyedropper01.jpg

Next, just click in the area of your artwork that you want to be transparent.
In your case it sounds like you would pick the corner. See here:
http://joedasilva.com/work/misc/eyedropper02.jpg

A couple of things to note: Fireworks will dither rounded edges, so make your
background as close to the color that will appear behind it as possible before
using the eyedropper. If you had a white canvas, and then eye-dropped the
white, and then placed the image on a black background, you'd see some white
leftover around the edges. Once you start playing around with this more the
details should become more clear ;-)

thanks for both your input. I will try what you said Joey and will also try and link the .png file