\begin{figure}
  \centering
  \begin{subfigure}[t]{0.45\textwidth}
    \includegraphics[width=\linewidth, height=20em]{example-image-a}
    \caption{taller image}
  \end{subfigure}
  \begin{subfigure}[t]{0.45\textwidth}
    \includegraphics[width=\linewidth, height=15em]{example-image-b}
    \caption{shorter image}
  \end{subfigure}
  \caption{two images}
\end{figure}