Defacement
Defacement is a term used in heraldry and vexillology to refer to the addition of a symbol to a background. Thus, for example, the blue canton of the national flag of the United States is correctly described as a blue background defaced by fifty white five-pointed stars.
The term does not imply any insult to the original background, as the symbol is usually used for differentiation of the shield or flag from that of another owner.
In computing, web site "defacement" is usually the substitution of the original home page by a hacker. The best-known defacement archive is http://www.zone-h.org which monitors web defacements but also web intrusion at any level (not only the homepage). Defacement is generally meant as a kind of electronic graffiti while recently it has become a means to spread messages from politically motivated "cyber protesters".