\begin{frame}
  \small
  
  \newcommand{\rc}{\tikz[baseline=-2pt]{\node{\includegraphics[width=6mm]%
  {../graphics/red.jpg}};}}
  \newcommand{\bc}{\tikz[baseline=-2pt]{\node{\includegraphics[width=6mm]%
  {../graphics/blue.jpg}};}}
  \newcommand{\gc}{\tikz[baseline=-2pt]{\node{\includegraphics[width=6mm]%
  {../graphics/green.jpg}};}}
  
  \begin{definition}
  \smallskip
  TRS $\RR$ over signature $\FF$ is
  \alert{string rewrite system} (\alert{SRS}) if $\FF$ consists
  of unary function symbols
  \end{definition}
  
  \bigskip
  
  \begin{example}<2->
  \vspace{-1ex}
  \begin{xalignat*}{2}
  \alt<3>{\rc\gc}{\rc(\gc(x))} &~\to~ \alt<3>{\bc\bc}{\bc(\bc(x))} &
  \alt<3>{\gc\rc}{\gc(\rc(x))} &~\to~ \alt<3>{\bc\bc}{\bc(\bc(x))}
  \\
  \alt<3>{\bc\rc}{\bc(\rc(x))} &~\to~ \alt<3>{\gc\gc}{\gc(\gc(x))} &
  \alt<3>{\rc\bc}{\rc(\bc(x))} &~\to~ \alt<3>{\gc\gc}{\gc(\gc(x))}
  \\
  \alt<3>{\gc\bc}{\gc(\bc(x))} &~\to~ \alt<3>{\rc\rc}{\rc(\rc(x))} &
  \alt<3>{\bc\gc}{\bc(\gc(x))} &~\to~ \alt<3>{\rc\rc}{\rc(\rc(x))}
  \end{xalignat*}
  \end{example}
  
\end{frame}