minimal example for http://tex.stackexchange.com/a/64665/929

sig_overlay.tex

% curl -L -o signature.png \
%  'https://upload.wikimedia.org/wikipedia/commons/thumb/6/62/Beethoven_Signature.svg/150px-Beethoven_Signature.svg.png'
% compile twice: pdflatex sig_overlay.tex
\documentclass{article}
\usepackage{blindtext}
\usepackage{graphicx}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[remember picture,overlay]
  \node [xshift=150mm,yshift=-80mm]
        at (current page.north west) {\includegraphics{signature}} ;
\end{tikzpicture}
\blindtext
\end{document}