Graphics can be very easily overdone.
On the other hand, a site with nothing but text is unappealing to most of us. Some images look better if their background color matches the browser window, making the image appear to float in the window. You can't do this by setting the background color of the image, since you can't control how people will configure their browsers. You can control this by using GIF89a images, which have the ability to mark a single color in the colormap as transparent, forcing the browser to use its background color for those pixels in the image. Anti-aliasing is a technique used in graphics construction to minimise the 'jaggies' that you can see on lines that are not perpendicular or horizontal.