Most low end desktop scanners support 24 bit color. This set of colors displays equally well on a Mac or on a PC and is handled well on older systems that only support 256 colors. This palette should also help one to display graphics properly for presentations as well. The Web safe palette is built in to most newer versions paint programs. The last of the three important things to consider in handling Web and presentation graphics is resolution.
Resolution refers to how many pixels display on a monitor screen.
It is a good idea to create graphics for 72 ppi since many systems only handle this. It is not worthwhile to use a higher ppi as that will make the graphics files larger. Most paint programs and scanners allow you to set the ppi. I use 72 ppi for all my graphics. They look good even on monitors with a higher ppi.