beamer_twoPic1.tex

% 横に並べる
% cf. http://tex.stackexchange.com/questions/232259/adding-captions-to-aligned-images-in-beamer

\documentclass{beamer}

\begin{document}

\begin{frame}

\begin{columns}[onlytextwidth]
\begin{column}{.45\textwidth}
\begin{figure}
  \includegraphics[width=\textwidth]{example-image-a}
  \caption{First image}
\end{figure}
\end{column}
\hfill
\begin{column}{.45\textwidth}
\begin{figure}
  \includegraphics[width=\textwidth]{example-image-b}
  \caption{Second image}
\end{figure}
\end{column}
\end{columns}

\end{frame}

\end{document}

beamer_twoPic2.tex

\documentclass{beamer}
\usepackage{graphicx}

\begin{document}

\begin{frame}[plain]
\makebox[\linewidth]{% 横に2枚並べる
  \includegraphics[width=0.5\paperwidth,height=\paperheight]{cat1}%
  \includegraphics[width=0.5\paperwidth,height=\paperheight]{cat2}}
\end{frame}

\begin{frame}[plain]
\makebox[\linewidth]{% 縦に2枚並べる
  \includegraphics[width=\paperwidth,height=0.5\paperheight]{cat1}}\\\nointerlineskip
\makebox[\linewidth]{%
  \includegraphics[width=\paperwidth,height=0.5\paperheight]{cat2}
}
\end{frame}

\end{document}