The quality option decreases the amount of blurring and artifacts in an image by increasing the size of the background layer. This option is especially effective when you encode images that contain photographs. A higher quality improves the appearance of the photographs at the expense of an increased file size.
If your primary purpose is to archive a copy of an image, assign a very high value (95) to the quality option to produce nearly lossless results. If your primary purpose is to publish an image on the Web, a value of 70 or 75 is generally effective. Values lower than 70 should be used only when storage requirements are more important than visual appearance, or when the background layer is unvarying (for example, colored paper).
Supporting commands:
documenttodjvu, phototodjvu