
\usepackage{subfigure}

\begin{document}

\begin{figure}[ht]
\centering
\subfigure[Uniform Kernel]{
\includegraphics[scale=1]{uniform-kernel.png}
\label{fig:uniform-kernel}
}
\subfigure[Triangular Kernel]{
\includegraphics[scale=1]{triangular-kernel.png}
\label{fig:uniform-kernel}
}
\subfigure[Epanechnikov Kernel]{
\includegraphics[scale=1]{epanechnikov-kernel.png}
\label{fig:epanechnikov-kernel}
}
\subfigure[Gaussian Kernel]{
\includegraphics[scale=1]{gaussian-kernel.png}
\label{fig:gaussian-kernel}
}