Beamer Bibliography Icon

\documentclass{beamer}

\begin{document}

\begin{frame}
\begin{thebibliography}{9}
\setbeamertemplate{bibliography item}[online]
\bibitem{A} ItemA
\setbeamertemplate{bibliography item}[book]
\bibitem{B} ItemB
\setbeamertemplate{bibliography item}[article]
\bibitem{C} ItemC
\setbeamertemplate{bibliography item}[triangle]
\bibitem{D} ItemD
\setbeamertemplate{bibliography item}[text]
\bibitem{E} ItemE
\end{thebibliography}
\end{frame}

\end{document}

Of course, you can also use your own image(s):
\documentclass{beamer}

\begin{document}

\begin{frame}
\begin{thebibliography}{9}
\setbeamertemplate{bibliography item}{\includegraphics[width=1.5em]{ctanlion}}
\bibitem{A} CTAN lion drawing by Duane Bibby.
\end{thebibliography}
\end{frame}

\end{document}