Exact Creates a palette using the exact colors appearing in the RGB image--an option available only if the image uses 256 or fewer colors. the reason that it's not an option is that you have designed / painted with more than 256 specific colours. an exact file / .gif can't hold anymore than that much info. I don't know why you have chosen to change the "image mode" when you can just save for web. I like those "local" ones , i'm not sure which one is the best. I have a feeling it's selective.