From Help:
Optimize transparency in GIF and PNG images
Transparency makes it possible to create nonrectangular images for the web. Background transparency preserves transparent pixels in the image. This allows the background of the web page to show through the transparent areas of your image. Background matting simulates transparency by filling or blending transparent pixels with a matte color that can match the web page background. Background matting works best if the web page background is a solid color and if you know what that color is.
Use the Transparency and Matte options in the Save For Web & Devices dialog box to specify how transparent pixels in GIF and PNG images are optimized.
(GIF and PNG‑8) To make fully transparent pixels transparent and blend partially transparent pixels with a color, select Transparency and select a matte color.
To fill fully transparent pixels with a color and blend partially transparent pixels with the same color, select a matte color and deselect Transparency.
(GIF and PNG‑8) To make all pixels with greater than 50% transparency fully transparent and all pixels with 50% or less transparency fully opaque, select Transparency and select None from the Matte menu.
(PNG‑24) To save an image with multilevel transparency (up to 256 levels), select Transparency. The Matte option is disabled since multilevel transparency allows an image to blend with any background color.
Note: In browsers that do not support PNG‑24 transparency, transparent pixels may be displayed against a default background color, such as gray.
To select a matte color, click the Matte color swatch and select a color in the color picker. Alternatively, select an option from the Matte menu: Eyedropper (to use the color in the eyedropper sample box), Foreground Color, Background Color, White, Black, or Other (to use the color picker).
There’s lots more too!
