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.
TikZ
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 baseline option). You will need to use the at (<x>,<y>) 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.
textpos
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} % horizontal unit
\setlength{\TPVertModule}{1cm} % vertical unit
...
\begin{textblock}{WIDTH}(OFFSET_X,OFFSET_Y)
CONTENT
\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}%
A notice that was put in the page using coordinates
\end{minipage}
}%
\end{textblock}%
You could use the overprint environment so that the same space is reserved on all slides.
\begin{frame}
\frametitle<1>{Title for slide 1}
\frametitle<2>{Title for slide 2}
\frametitle<3>{Title for slide 3}
\begin{overprint}
\onslide<1> introductory text for first slide
\onslide<2> introductory text for second slide
\onslide<3> introductory text for third slide
\\can also be longer
\end{overprint}
\begin{overprint}
\onslide<1>\centerline{\includegraphics[width=<length>]{filename1}}%
\onslide<2>\centerline{\includegraphics[width=<length>]{filename2}}%
\onslide<3>\centerline{\includegraphics[width=<length>]{filename3}}%
\end{overprint}
\end{frame}
Another simple idea is to add a \vfill between text and graphic to push both apart. Then the graphic will always be at the bottom. A third alternative is to use the textpos package to place the graphic using absolute coordinates.
Best Answer
You are mixing two graphic systems (tikz and the original picture environment of latex). With tikz you should place graphics (and text) with nodes: