If the image you are compressing contains photographic regions or regions with many colors or smooth gradients you may need to add dithering to improve the look of these regions. To add dithering drag the dithering slider to the right. A value of 50% works for most images. Dithering will make the image larger so use it sparingly. After you have applied dithering you may be able to further reduce the number of colors. If you need to use a lot of dithering you may find a MagiCompressed JPEG works better.
You can use the 
button to remove all dithering or smoothing.
Improve Palette
In order to speed things up Image Optimizer makes some approximations when selecting a palette. If you want a palette of maximum accuracy press the Improve palette button 
at the end of your optimization. Image Optimizer will then re-calculate the palette in greater detail. This may take some time unless you have a fast machine.
This can be useful with highly compressed images that have a background color. Image Optimizer may assign the background to a compromise color that is not quite right. The HiQ button will often fix this problem.
Step 4 (optional): Modify regions (Simple)