Figure, side by side

Use "subfigure", or use below:

\begin{figure}
\begin{center}
\leavevmode
\begin{tabular}{cc}
\includegraphics[height=0.35\textwidth]{bifurcation.png} & \includegraphics[height=0.35\textwidth]{liver.PNG}
\end{tabular}
\end{center}
\caption{The two landmarks tracheal bifurcation (left) and liver top (right) used in the experiment.}
\label{fig:score_map_comparison1}
\end{figure}