It is recommended that version GIF89a is used because it allows for one of the colors in the image to be transparent, usually the background color. GIF is most effective with images that have areas of single colors. As the name implies, the JPEG format is for use with photographs. This format supports the millions of colors that may make up a photograph. Backgrounds Graphics may also be used as a background to a Web page.
Backgrounds are graphics that are tiled, or repeated, across the page.
Make sure the graphic is appropriate as a background and does not distract from the page elements.