They create the environment of your site and provide unity and continuity as visitors move through it. Images perform several functions on a web page. As with anything, advantages are countered by disadvantages. Pages with images take longer to download. The biggest disadvantage to placing images on your web site is something completely beyond your control - images are only as good as the monitor that displays them.
Resolution is controlled completely by the hardware and the visitor.
While there are no absolute universal standards, selecting your palette from the web-safe color palette will give you the most control and enable you to anticipate how colors will be displayed on most browsers and in different environments. There are many resources for imagery available for the web designer. Some generous designers have made images available free of charge on the WWW, requesting only credit for their work. Clip art also comes bundled with most word-processing applications. Certain fonts can be used to produce navigation icons.