Beamer, transparent background colour

Dear all,
I like to have in beamercolorbox transparent background colour. This I try to achieve with naive solution:

\setbeamercolor{block body}{fg=black,bg={yellow!10,\opaqueness{.5}} }

but it is to naive an so it doesn't work. Do any one better idea how to modified minimal (dummy) example below, that it will has a described features?


regards, Zarko

p.s. I attach the background image example (a helicopter)




\documentclass{beamer}
%---------------------------------------------------------------%
\usepackage[slovene,english]{babel}
\usepackage[cp1250]{inputenc}\usepackage[T1]{fontenc}
\usepackage{graphics,tikz}
%---------------------------------------------------------------%
\defbeamertemplate{background}{image}[1][]{
\pgfdeclaremask[matte={1 1 1}]{background image}{#1}
\pgfdeclareimage[interpolate=true,
height=.8\textheight,
mask=background image]{background image}{#1}
\begin{pgfpicture}{0cm}{0cm}{\the\paperwidth}{-\the\paperheight}
\pgfputat{\pgfxy(6.4,-4.8)}
{\pgfbox[center,center]%
{\pgfuseimage{background image}}}
\end{pgfpicture}%
}
\setbeamertemplate{background}[image][slika3]
\setbeamercolor{block body}{fg=black,bg=yellow!10}

%---------------------------------------------------------------%
\begin{document}
%---------------------------------------------------------------%
\begin{frame}
\framesubtitle{Expertise with OPNET}
\framesubtitle{Test of back colour transparencies}
%------------------------------------------------------------ 9
\begin{beamercolorbox}[sep=0pt,wd=80mm]{block body}
\begin{itemize}
\item Modeliranje, simuliranje in vrednotenje TK omrežij
\item Merjenje in modeliranje telekomunikacijskega prometa
\item Ekspertni sistemi za optimiranje radijskih omrežij
\item Kakovost storitev v usmerjevalnih protokolih
\item Komunikacije v sprotnem času
\item Storitve s programskim paketom OPNET
\end{itemize}
\end{beamercolorbox}
\end{frame}
%---------------------------------------------------------------%
\end{document}

JPEG image