latex subfigure align top
\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}