Help with beamer environments

[Vikas Rawal <vikaslists@agrarianresearch.org>]

I am trying to use textpos to position images at specific location on
a frame.

I would like something like this in the beamer export:

\begin{textblock}{10}(3,3) \visible <2-> {
\includegraphics[width=.9\linewidth]{scatterplot2.png}
} \end{textblock}

I have defined the following beamer environment.

(add-to-list 'org-beamer-environments-extra
             '("textpos1" "w" "\\begin{textblock}{%h}(3,3) \\visible %a {" "} \\end{textblock}"))

The problem is (3,3) is fixed in the above specification. How can I
specify it for a given headline? I have tried various ways of
generalising this but nothing seems to work.

For example, if I use the following:

(add-to-list 'org-beamer-environments-extra
             '("textpos1" "w" "\\begin{textblock}%h \\visible %a {" "} \\end{textblock}"))

and write the headline as {10}(3,3), I get \{10\}(3,3) in beamer
export rather than {10}(3,3).

How does one use the escape %o? I have looked through ox-beamer.el,
worg and mailing list archives, but could not find a clear
explanation. Will be grateful for a pointer.

Vikas