\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*}
