\documentclass{beamer}
\usepackage{tikz}

\begin{document}

\begin{frame}
  \begin{tikzpicture}
    \alt<2->{\node[opacity=1]  {\includegraphics[width=.5\textwidth]{../img/logoUJI_half}};}
            {\node[opacity=.15] {\includegraphics[width=.5\textwidth]{../img/logoUJI_half}};}
  \end{tikzpicture}
\end{frame}

\end{document}

