gif/Fireworks/Dreamweaver Q

Being totally new to Images & Macromedia etc, I guess this is really simple to you people.

When I create a rollover image in Dreamweaver (to be placed within a table cell) the on/off images I nominate have a thick whilte ugly border around them.

I trace this back to Fireworks where I use the Rectangle tool to create a shape, then place text in it, then Export as a gif. I THINK my trouble stems from the gif taking not only the object I drew but the white central drawing area (inside the 'canvas')? I cannot find a way to either :
Delimit the borders of the shape I want to be the gif, or
'Flood' the central drawing space with my shape & colour or
Reduce the white drawing area to the borders of my drawing.
...... Then I can presumable export as a gif and 'import' it beautifully into Dreamweaver. Fireworks Help has zero info on this area ..... can anyone help ?????