Since conversion from one format to another causes loss of clarity, and possible resizing of the image, try to avoid conversion when possible. As a general rule, any images used on the WWW should be less than 50K in size. The quality and file storage size of a scan are partially defined by the mode, resolution, and scale you specify, and whether you crop the picture. If scanning Gray Scale of Halftone images, this converts the entire image to black and white. In older newsprint you could see small dots of varying sizes creating the illusion of gray scale.
Comic books use larger screen dots.
Photo - Gray Scale, Continuous Tone . But this option costs in terms of file size. If you only intend to use part of an image, crop it to keep the file size down. Increasing the size of an image after scanning may cause it to blur, so always scan it at the desired size. If you are scanning an image to make it into a slide, be sure to save the image size with a height to width ratio of 2 to 3.