A major problem with using images on Web sites is that
images take much longer to load than text. So you have
to choose your image formats carefully.
Two image formats are most commonly used and
supported on the Internet: GIF and JPEG. These formats use different methods to reduce the size
of an image. Normally you should use JPEG images for
photographs and other images that contain significant
color variation. GIF images are well suited for images
that contain sharp lines such as icons, buttons, or
Web site banners.