Image size is a major problem, both physical size in inches on a screen and memory size in kilobytes. The trick is to optimize both, to find the greatest physical size and the lowest cost in memory. As a general rule, in-line graphics (graphics automatically loaded along with text) should not be more than 150 pixels x 150 pixels, should be scanned at the lowest possible dpi (dots per inch) consistent with legibility, and should be stored as .gif images. Off-line graphics, those kept off the screen until the reader elects to view them, can be larger in physical size and memory. They should probably never be larger than about 400 x 600; these can be stored as compressed .jpeg images.