You have to center the picture with respect to the math axis: but it's best to define a personal command for this
\documentclass{article}
\usepackage{graphicx}
\newcommand{\bracedincludegraphics}[2][]{%
\sbox0{$\vcenter{\hbox{\includegraphics[#1]{#2}}}$}%
\left\lbrace
\vphantom{\copy0}
\right.\kern-\nulldelimiterspace
\underbrace{\box0}}
\begin{document}
$r\bracedincludegraphics{image1}$
\end{document}
The command \bracedincludegraphics
accepts the same options as \includegraphics
.
I set the image in a box for later use, centered with respect to the math axis (that's the main problem with your try, as the image has all height and no depth, and \left
assumes that it has to cover as much over the math axis as below).
Then I use the box to determine the size of the left brace with a \vphantom
, close with \right.
and kern back a bit; only after this I typeset the image with the underbrace.
EDIT
In order to add a small clearance also between the underbrace and the image, the following modified macro should help:
\newcommand{\bracedincludegraphics}[2][]{%
\sbox0{$\vcenter{\hbox{\includegraphics[#1]{#2}}}$}%
\left\lbrace
\vphantom{\copy0}
\right.\kern-\nulldelimiterspace
\underbrace{\vrule width0pt depth \dimexpr\dp0 + .3ex\relax\box0}}
The .3ex
should be what's needed, but it depends on the font actually used for math.