They are not suitable for photos, watercolours, or photo-like illustrations. Progressive JPEGs work the same way as interlaced GIFs. This will help speed up download time.
All graphic elements add to the download time of a page.
Keep the file size of your graphics to the minimum without loss of quality, and find a balance between visual interest and download time. Pages that are read exclusively over the Intranet can have larger graphic file sizes because they don't have to be transferred via modem. All images should contain alt labels to make pages understandable to readers with their images turned off or with text-only browsers. So readers with text-only browsers can access all links, image maps should be mirrored on the page by corresponding text links, perhaps in the form of a bar with text on it. Restrict the use of image maps to true map-like applications.