2 of 3 Disappeared backslash

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}

enter image description here

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.