Add four images on a beamer frame

Question

What is the best way to add four images on a beamer frame? I am using

\documentclass[slidestop,compress,11pt,xcolor=dvipsnames]{beamer}
\usefonttheme[onlymath]{serif}
\definecolor{LHCblue}{RGB}{4, 114, 255}
\usecolortheme[named=LHCblue]{structure}
\usepackage[bars]{beamerthemetree} % Beamer theme v 2.2
\usepackage{multicol}
\usepackage{lmodern}
\usepackage{lipsum}
%\usepackage{graphicx}
\usepackage{marvosym}

\begin{document}
\begin{frame}
\begin{frame}
\begin{columns}
\begin{column}{.49\textwidth}
\centering
\begin{figure}[H]
\includegraphics[width=5cm,height=3.5cm]{lab1}
\end{figure}
\hspace{0.7cm}
\centering
\begin{figure}[H]
\includegraphics[width=5cm,height=4cm]{lab2}
\end{figure}
\end{column}%
\hfill%
\begin{column}{.49\textwidth}
\includegraphics[width=5cm,height=4cm]{lab3}\\
\includegraphics[width=5cm,height=4cm]{lab4}
\end{column}%
\end{columns}
\end{frame}
\end{frame}

\end{document}

The result is not the best...

Any possible ideas will be welcomed!

Answer 1

You don't really need the figure environment since you are not providing captions (beamer, in any case, deactivates the floating mechanism); so you can simply say:

\PassOptionsToPackage{demo}{graphicx}
\documentclass[slidestop,compress,11pt,xcolor=dvipsnames]{beamer}
\usefonttheme[onlymath]{serif}
\definecolor{LHCblue}{RGB}{4, 114, 255}
\usecolortheme[named=LHCblue]{structure}
\usepackage[bars]{beamerthemetree} % Beamer theme v 2.2
\usepackage{multicol}
\usepackage{lmodern}
\usepackage{lipsum}
\usepackage{marvosym}

\begin{document}

\begin{frame}
\begin{columns}[t]
\column{.5\textwidth}
\centering
\includegraphics[width=5cm,height=3.5cm]{lab1}\\
\includegraphics[width=5cm,height=4cm]{lab2}
\column{.5\textwidth}
\centering
\includegraphics[width=5cm,height=4cm]{lab3}\\
\includegraphics[width=5cm,height=4cm]{lab4}
\end{columns}
\end{frame}

\end{document}

Of course, you might need to adjust the width and or height of your images.

The line

\PassOptionsToPackage{demo}{graphicx}

was only included to replace actual figures with black rectangles; delete that line from your actual code.

On a side note, the slidestop class option is obsolete, so you should load the class using the t option instead:

\documentclass[t,compress,11pt,xcolor=dvipsnames]{beamer}