From: Daniel Piechnick
Hi Shannon. I'm sure you're aware of the limitations of gifs, in that they can't have partially transparent pixels. They can only be fully solid, or totally transparent. What happens is that Photoshop finds any partly transparent pixels at the edge of your image, and combines them with a "Matte" colour to make them solid. When you "Save for Web", choose a matte colour that isn't white - make your matte colour the same colour as the background that the image is going to be on.