Most sites on the web today use images or graphics. The two terms are generally interchangeable in a web context. (Sometimes a distinction is drawn between photographic "images" and line-art "graphics." We use the terms interchangeably.)
The most important principles for images/graphics on EMU web sites are that they should be relevant, professional, optimized for the web, and legal.
The principles and guidelines here deal primarily with static images, but the same principles generally apply to images in flash animations, video clips and other multimedia presentations.