How can I position an image in an arbitrary position in beamer?

When I used such an application, I used either TikZ (my favorite), or, before knowing of TikZ, I used the textpos package.

I will briefly discuss both solutions.

I suggest to use a TikZ \node with overlay option.

NOTE: there are two ways to use TikZ here: one with RELATIVE positioning (the approach discussed below) and one with ABSOLUTE positioning (please refer to the TikZ manual for such a task).

The overlay means that it has no bounding box, i.e. it won't affect the existing layout. I wrote a short-cut macro around to simplify the generation of such a node. To use it, place the following code into you beamer document:

\tikzset{
  every overlay node/.style={
    draw=black,fill=white,rounded corners,anchor=north west,
  },}
% Usage:
% \tikzoverlay at (-1cm,-5cm) {content};
% or
% \tikzoverlay[text width=5cm] at (-1cm,-5cm) {content};
\def\tikzoverlay{%
   \tikz[baseline,overlay]\node[every overlay node]
}%

So, the basic idea is a \node[overlay] which aligns well within the usual text (the baselineoption). You will need to use the at (,) positioning to actually place it. The placement will be relative to the embedding text. Keep in mind that you may hide text with this thing (which is the purpose of the every overlay node style as you guessed correctly). Note that text later in the document is always typeset after earlier text passages. In other words: make sure the \tikzoverlay command comes after things you want to hide.

An example could be

Here is the text which will probably be hidden by the overlay.\tikzoverlay[text width=6cm] at (9.3cm,5cm) {
  \begin{itemize}
  \item \emph{Derive subclass} from \texttt{GetOptWrapper}
  \item one \emph{variable definition} per option
  \item \emph{Default Values}
  \end{itemize}
};

You can safely combine this with beamer's \only<2> methods.

The second promised approach uses the textpos package. Since I think that this approach is weaker and less configurable than the one above, I simply copy-paste from my personal notes:

According to my old notes, the general usage is:

\usepackage{textpos}
\setlength{\TPHorizModule}{1cm} % Horizontale Einheit
\setlength{\TPVertModule}{1cm} % Vertikale Einheit
...\begin{textblock}{WIDTH}(OFFSET_X,OFFSET_Y)
INHALT\end{textblock}
...

Example with the package configuration above:

\definecolor{boxcol}{gray}{0.89}
\begin{textblock}{3}(8,-3)
\fcolorbox{black}{boxcol}{%
\begin{minipage}{\textwidth}
\setlength{\parindent}{0pt}%
\setlength{\parskip}{0.1cm}%
Ein Hinweis, der mittels Koordinatenangabe plaziert wurde\end{minipage}
}%
\end{textblock}%