documentclass[12pt,a4paper]{article}
usepackage{eso-pic}

begin{document}
null
letpositionAtPageCenter
    AddToShipoutPictureBG{%
        position{%
            put(0,0){paper center waterprint}
        }
    }
%
letpositionAtPageLowerLeft
    AddToShipoutPictureBG{%
        position{%
            put(20,20){lower left waterprint}
        }
    }
end{document}