Graffiti on the web? Permanently is it possible?

It is possible to hack and change a page, thus add any kind of text (including graffiti). Don't ask me how cause I don't know, I only know that it's possible. However it's almost impossible to make it permanent. Simply because it is very easy for the original owner to change the files to put it back to the original file...

The only way graffiti is permanent, is if you put it in a page where nobody notices it (and can't tell the owner). But that defeats the whole purpose of graffiti right?