I turned on the dither to diffusion but it doesn do anything. What is that for?
By default the dither is set to 0%. In order to use the dither setting, you need to give a percentage to the dither up to 100% in the box to the right of the dither setting. The dither setting can be helpful when you have to save as a GIF for transparency or animation and the image is a photo or has glows, drop shadows or gradients.