Depending on their use they can make or break a web site. Web designers need to be careful that visitors are not bombarded with graphic images, that backgrounds do not interfer with readability and that colors and images do not draw attention away from the content of the page. There are two kinds of graphics, Raster and Vector. Raster graphics are also called bitmaps. They are tiny dots which when viewed together form an image.
Vector graphics are a little more complicated.
They are made through a series of equations that tell the computer how much area to color in. They are more precise and take up less memory but vector graphics cannot be read by all browsers. For now I will only be refering to Raster graphics better known as bitmap graphics. There are many great graphics sites on the web. Most of these offer free graphics that you can easily download.