Make sure your graphics are consistent with both the tone and the look and feel of your site. For creating and manipulating images, get a graphics program, like the excellent shareware Paint Shop Pro, or the more expensive but industrial strength Adobe PhotoShop.
Many sites provide free graphics and elements to use.
Use graphical elements such as illustrations, bullets, dividers, etc. only when they truly add value. And reuse the same components, they will be cached and your next page will load quicker. Since photos are often bandwidth hogs consider providing a small thumbnail or clipart link to the larger photo on its own page. Small buttons in a table may load faster as a navigation device than an image map will. A ' browser-safe ' palette of 216 colors will look the same on PCs and Macs.