Remove halos
When
you make the canvas color transparent on an image that was previously anti-aliased,
the pixels from the anti-aliasing remain. Then when you export (or in
some cases, save) the graphic and place it on a web page with a
different background color, the perimeter pixels of the anti-aliased
object may appear as a halo, which is especially noticeable on a
dark background.
Prevent halos in Fireworks PNG files and imported Photoshop files
Do one of the following:
Set the
Canvas Color in the Property inspector or the Matte Color in the Optimize
panel to the color of the target web page background.
With the object you want to export selected, select Hard
from the Edge pop-up menu in the Property inspector.
Remove a halo manually from a GIF or other graphic file
With the file open in Fireworks, click the Preview,
2-Up, or 4-Up button at the upper left of the document window. In
2-Up or 4-Up view, click a view other than the original.
In the Optimize panel, select Index Transparency from the
Transparency pop-up menu.
Click the Add Color To Transparency button 
and
click a pixel in the halo.
All pixels of the same color are
removed in the preview.
If the halo is still there, repeat step 3 until the halo
is gone.
PDF (adobe.com)
