Download
http://www.libpng.org/pub/png/img_png/stefan_255_rgba.png
(which has a transparent background) and run it through
png2pdf as follows:
png2pdf -i stefan_255_rgba.png stefan_255_rgba-i.pdf
png2pdf -a stefan_255_rgba.png stefan_255_rgba-a.pdf
png2pdf stefan_255_rgba.png stefan_255_rgba-neither.pdf

Then run the following through pdflatex (pdfTeXk,
Version 3.141592-1.40.3 (Web2C 7.5.6))

\documentclass{beamer}
\mode<presentation>
{
  \usetheme{Madrid}
  \setbeamercovered{transparent}
}

\usepackage{ifpdf}
\usepackage{graphicx}
\usepackage{color}
\ifpdf
\hypersetup{%
  pdfpagemode=FullScreen,
  pdfpagetransition={Wipe /Di 0 /D 0.5}
}
\fi

\title[Beamer and png2pdf]{Using png2pdf with the
beamer class}
\author[Krause]{D.~Krause}
\subject{png2pdf}

\begin{document}
\beamertemplateshadingbackground{yellow!50}{magenta!50}

\begin{frame}
\frametitle{NO Image over background}
This is some text
\end{frame}

\begin{frame}
\frametitle{Original stefan... png image over background}
\includegraphics{stefan_255_rgba.png}%
\end{frame}

\begin{frame}
\frametitle{png2pdf -a stefan... image over background}
\includegraphics{stefan_255_rgba-a.pdf}%
\end{frame}

\begin{frame}
\frametitle{png2pdf -i stefan... image over background}
\includegraphics{stefan_255_rgba-i.pdf}%
\end{frame}

\begin{frame}
\frametitle{png2pdf stefan... over background}
\includegraphics{stefan_255_rgba-neither.pdf}%
\end{frame}

\end{document}

Now look at the resulting PDF (attached) with Acroread
7.0.9 or 8.1.1 on Linux.  On the page which includes
the PNG and also on the page which includes the png2pdf
-a version,
the other colours on the page are messed up somewhat.

This also happens on Acroread 8.? running on Windoze xp
under vmware, although it seems a bit less noticeable.

There is no apparent problem when looking at the file
with xpdf or kpdf (Slackware 12.0 Linux).

It's not clear to me whether this is an acroread bug or
a pdftex bug, but Martin Schröder suggested I open a
bug here.

Cheers.
              Jim