Web Design in a Nutshell, 3rd Edition

Use the Matte color tool

If you are using Photoshop/ImageReady or Fireworks, the best way to prevent a halo is to use the Matte color tool. The tool requires that you start with a layered file that already contains transparent areas. In other words, the image must not have already been “flattened.” The parts of the layered image that are transparent will remain transparent when exported to GIF format .

In the tool’s optimization palette, simply set the Matte color to the same color as the background of the page on which the GIF will appear (Figure 29-8). When the GIF is exported with Transparency selected, the anti-aliased edges of the image blend with the selected Matte color. That blend ensures there will be no halo.