For photographs, drawings
and all colour images scan at 300dpi or higher. For black & white line art use 1200dpi.
scan your material at the size at which it will be printed. Enlarging images in your
artwork can seriously degrade quality.
In other words don't scan your stuff and then enlarge it in your
layout program! This is another reason why you should not use images that are
lifted from websites; they are usually only 72dpi and will look very blurry
indeed when printed on a 'litho' printing press.
Pictures from digital
cameras are designed to be printed and generally reproduce very well
(much better than the equivalent film camera). As always though, the better the
equipment, the better the result.
As a rough guide divide the image dimensions (given in pixels)
by 300 to give the approximate 'litho' printing size. For example - a picture
of 1800 x 1200 pixels would be adequate for a printed size of up to 6in x 4in.