begin{figure}
begin{center}
hspace{-0.25in}includegraphics[width=0.4textwidth]{img/a.png}~
hspace{-0.75in}frame{includegraphics[width=0.175textwidth]{img/b.jpg}}
end{center}
caption[stuff]{label{fig:stuff}Some stuff.}
end{figure}

begin{figure}
begin{center}
hspace{-0.25in}includegraphics[width=0.4textwidth]{img/a.png}~
hspace{-0.75in}vspace{0.25in}frame{includegraphics[width=0.175textwidth]{img/b.jpg}}
end{center}
caption[stuff]{label{fig:stuff}Some stuff.}
end{figure}