\usepakage{subfig}


\begin{figure*}[!htb]
  \centering
    \subfigure[Derm101 data distribution]{\includegraphics[width=0.5\textwidth]{derm101_stat.png}} &
    \subfigure[Skin-100 distribution]{\includegraphics[width=0.44\textwidth]{skin100_stat.png}} \    \subfigure[Derm101 data distribution]{\includegraphics[width=0.5\textwidth]{derm101_stat.png}} &
    \subfigure[Skin-100 distribution]{\includegraphics[width=0.44\textwidth]{skin100_stat.png}} \  \caption{Geographic Distribution of Cell Towers, Number of Records, User Population and Data Volume}
    \label{fig:data_distribution}
    \vspace{0.2in}
\end{figure*}
 

LaTeX多圖合並代碼示例(subfigure)