Beamer class (presentation mode) in LaTeX - an example of subfloats

I'd thought I'd post an example of a subfloat (a subfigure, but subfig is the package and subfloat the command; the subfigure package and command are older, see http://www.faqoverflow.com/tex/1966.html and http://texblog.org/tag/subfigure/) in Beamer, a presentation class for LaTeX.

\begin{frame}

  \begin{figure}[t]
   \centering
   \captionsetup[subfloat]{labelformat=empty}
   \subfloat[]{\label{} \includegraphics[scale = 0.15]{Sentinel_Bradford_et_al_07.png}} 
   \subfloat[]{\label{} \includegraphics[scale = 0.15]{Hyperion_(3)_Olds_et_al_99.png}}\\
   \subfloat[]{\label{} \includegraphics[scale = 0.15]{Stargazer_Olds_et_al_99.png}}
   \subfloat[]{\label{} \includegraphics[scale = 0.15]{Quicksat_(2)_Bradford_et_al_04.png}}
%    \captionsetup{labelsep=period, labelfont=sc} %\caption{} \label{fig:}
  \end{figure}

\end{frame}

Screenshot:

These are some concept hypersonic vehicles.  I just used this as an ending slide for my MS defense presentation.

Clockwise beginning at top-right:

Sentinel:

Bradford, J. E., Olds, J. R., & Wallace, J. G. (2007). Concept Assessment of a Hydrocarbon Fueled RBCC-Powered Military Spaceplane. 54th Joint Army-NavyNASA-Air Force (JANNAF) Propulsion Meeting (JPM) and 5th Modeling and Simulation / 3rd Liquid Propulsion / 2nd Spacecraft Propulsion Joint Subcommittee Meeting. Denver, Colorado.

Hyperion:

Olds, J. R., Bradford, J. E., Charania, A. C., Ledsinger, L., McCormick, D. J., & Sorensen, K. L. (1999). Hyperion: An SSTO Vision Vehicle Concept Utilizing Rocket-Based Combined Cycle Propulsion. 9th International Space Planes and Hypersonic Systems and Technologies Conference and 3rd Weakly Ionized Gases Workshop. Norfolk, VA.

Quicksat:

Bradford, J. E., Charania, A., Wallace, J., & Eklund, D. R. (2004). Quicksat: A Two-Stage to Orbit Reusable Launch Vehicle Utilizing Air-Breathing Propulsion for Responsive Space Access. Space 2004 Conference and Exhibit. San Diego, California.

Stargazer:

Olds, J. R., Ledsinger, L., Bradford, J. E., Charania, A., McCormick, D., & Komar, D. R. (1999). Stargazer: A TSTO Bantam-X Vehicle Concept Utilizing Rocket-Based Combined Cycle Propulsion. 9th International Space Planes and Hypersonic Systems and Technologies Conference and 3rd Weakly Ionized Gases Workshop. Norfolk, VA.