The simplest way is to use the adjustbox package:
\documentclass{article}
\usepackage[demo]{graphicx} % just to provide mock figures
\usepackage{adjustbox}
\begin{document}
\begin{tabular}{|c|c|}
\adjustbox{valign=t}{\includegraphics[width=1cm,height=.5cm]{x}} &
\adjustbox{valign=t}{\includegraphics[width=1cm,height=3cm]{x}} \\
\end{tabular}
\end{document}
Without any package,
\begin{tabular}{|c|c|}
\raisebox{\dimexpr-\height+\ht\strutbox}{\includegraphics[width=1cm,height=.5cm]{x}} &
\raisebox{\dimexpr-\height+\ht\strutbox}{\includegraphics[width=1cm,height=3cm]{x}} \ \end{tabular}
would do the same.
