The overpic.sty package can be found on any CTAN server or its mirrors. The README file reads:

This small LaTeX package defines the overpic environment which is
a combination between the usual picture environment and the includegraphics
command defined in graphicx. The resulting new picture environment has
automatically the same dimension as the included eps graphic. Now it is 
easily possible to put any LaTeX output on the graphic at defined position. 
A grid for orientation is also available.

This directory contains the following files:

README       - This file.
overpic.sty  - The definition of overpic. Required are graphicx.sty and 
               epic.sty.
opictest.tex - A demo that shows all the features. This file loads the eps
               graphic file golfer.ps which is part of ghostscript's 
               examples directory.
               
Please send comments and suggestions to niepraschk@ptb.de

Happy TeXing...

                                  Rolf Niepraschk, 7/97
                                                 
Check here for the example using the opictest.tex file from above.