how to vertically align a cell in a table?

Use \topincludegraphics defined as follows

\newcommand{\topincludegraphics}[2][]{%
  \raisebox{\dimexpr-\height+\ht\strutbox\relax}{\includegraphics[#1]{#2}}}

instead of \includegraphics; without \ht\strutbox there would be gaps, because each table row is always at least as high as a strut.

If you only have images in the cells you can use \raisebox to vertical shift all images below the baseline, so that they all get top aligned

\raisebox{-\height}{\includegraphics[width=7.3in]{imgX.pdf}}

However, if you have text in the same row it will be placed just above the images (baseline is on the top corner). In that more complicated case see How to vertically-center the text of the cells?.

\documentclass{article}
\usepackage{array}

\newsavebox\topalignbox
\newcolumntype{T}{%
  >{\begin{lrbox}\topalignbox
    \rule{0pt}{\ht\strutbox}}
  c
  <{\end{lrbox}%
    \raisebox{\dimexpr-\height+\ht\strutbox\relax}%
      {\usebox\topalignbox}}}

%%% or
%\newcolumntype{T}{%
%  >{\vtop\bgroup\vspace*{-\ht\strutbox}%
%    \hbox\bgroup\rule{0pt}{\ht\strutbox}}
%  c
%  <{\egroup\egroup}}

\begin{document}    

\begin{tabular}{|T|T|} \hline
  \rule{2cm}{3cm} & \rule{3cm}{4cm} \\ \hline
  \rule{4cm}{5cm} & \rule{2cm}{2cm} \\ \hline
  aabb & ccdd \\ \hline
\end{tabular}

\end{document}

This is somewhat complex. It is different with xport's solution since it ease the width calculation. The core code is the same as egreg's.

---

For vertical centering, it is better to use primitive TeX's \vcenter:

\documentclass{article}
\usepackage{array}
\newcolumntype{M}{>{$\vcenter\bgroup\hbox\bgroup}c<{\egroup\egroup$}}
\begin{document}
\begin{tabular}{|M|M|} \hline
\rule{2cm}{3cm} & \rule{3cm}{4cm} \\ \hline
\rule{4cm}{5cm} & \rule{2cm}{2cm} \\ \hline
\end{tabular}
\end{document}