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There are a number of legitimate reasons to scan images from books or magazines. Halftone images are reproduced in printed materials using screens or screening algorithms that allow us to see shades of gray or color. In color printing there are yellow, magenta, cyan and black dots that are printed on top of each other to “fool” the eye into seeing color or gray. A printing press is really a binary process. Either there is ink toner or there isn’t. We don’t use “fire engine” red ink or toner. We print the right density of yellow, magenta, cyan and black ink or toner on top of each other to create “fire engine red." If you scan an image from a book, magazine or newspaper you will scan in these dots which will cause a moiré pattern in your image. This article will point out ways to avoid this pattern or correct it after the fact.
Use the software on your scanner.
You will achieve the best result by correcting for moiré during the scanning of an image. The scanner is kind of a high resolution camera and although we can not usually see printing dots, the scanner will capture them very faithfully unless you program it otherwise. Most modern scanners have built-in software that allows them to eliminate or reduce the mount of moiré. You may want to reduce rather then eliminate the moiré if the pattern will interfere with the subject of the image. 
An image with a moiré pattern
This is an image from another eHow article on how to force bulbs by only1special1. Notice the pattern in the image. You can modify this image by using image editing software, however, it will not yield as good a result as dealing with the issue while scanning and may cause mottled colors and a loss of sharpness.
GIMP Despeckle Option
Open your image in an image editing program like GIMP. Under the filter menu, choose the DESPECKLE option. Use the adjustment sliders for black level, white level and radius until you have the best combination in the preview window. You cannot increase black level too much or the image will become too dark. The option works by adjusting the radius of the “speck” and the relative light and dark areas. Notice in the preview window we have reduced the pattern, but the image is a little softer in sharpness.
Before and After Image Editing
Notice the image on the left no longer has the moiré pattern. We have sacrificed a little sharpness and increased the mottle of the colors slightly. This can be further adjusted using the other tools of your image editing program.
