\documentclass{standalone}
\input{./tikzpreamble}
\begin{document}
    \begin{tikzpicture}
        \node[anchor=south west,inner sep=0] (image) at (0,0) {\includegraphics[width=\textwidth]{../data/theory/v18.pdf}};
        \begin{scope}[x={(image.south east)},y={(image.north west)}]
            \node[fill=white] at (.80, .80) {\includegraphics[width=.5\columnwidth]{../data/theory/v18_zoom.pdf}};
            \draw[draw=gray] (.381, .525) to (.613, .606);
            \draw[draw=gray] (.381, .642) to (.613, 0.990);
        \end{scope}
    \end{tikzpicture}
\end{document}