First GIF Images Gif images are made up of files consisting of 256 colors only. This is why they are best for images with large areas of the same color. But we can take it a step further. Just how you do this depends on which program you are using, but what we want to do is reduce those 256 down as far as we can go before the colors are unacceptable. As you preview the image, reduce the number and see what happens.
Some programs have palette and sampling methods to determine just what colors are left.
This is the best way to get the smallest file possible. Also, avoid dithering, a technique used in gif images to simulate colors between the 256 because it can increase file sizes in a big way. It's a bit more straightforward with jpeg images. Obviously, it can reduce the file, but can also do bad things to your image. That's why preview is important to getting the results you want.