I don't know of a package. But if you are willing to live under the constraint that the image cannot cover the surrounding text, much can be done with a simple \includegraphics. One must find signet images with a pure white background, of course.
\documentclass{article}
\usepackage{graphicx}
\usepackage{lipsum}
\usepackage[usestackEOL]{stackengine}
\begin{document}
Dear sir,
\lipsum[1-2]
Failure to take action will result in forfeiture of your property.
\bigskip\hspace{1.5in}Sincerely,\smallskip
\hspace{1.5in}\def\stackalignment{l}\stackunder[0pt]{%
\Longstack{Joe Schmoe\\Dept of Collections\\Washington D.C.}}
{\includegraphics{signet1}}
\end{document}
