Graphics must be in good taste and must add meaning to the page. Many people are turning off inline images or using character-based browsers such as Lynx. Make sure you meet their needs. Pictures and illustrations should only be used when they carry content themselves. This technique will give your client the chance to decide whether to view a particular graphic, and by linking graphics and putting them on several pages, each page will display much faster.
Larger graphics take longer to be displayed.
If a graphic takes a long time to be displayed it may take away from the effectiveness of the page. GIF is a standard graphic format for WWW pages. Don't use copyrighted graphics without permission. Graphics in magazines or newspapers are copyrighted. When in doubt, assume that the graphic is copyrighted, and get permission before using it.