In the provided example, a minus sign was left off the length .5\totalheight
. Graphical images are set above the baseline, when using \includegraphics
, whereas tabular
cells are centered. Thus, one must move the graphical image down by half its height to compensate.
\documentclass{article}
\usepackage{graphicx}
\begin{document}
\begin{table}[htbp]
\centering
\caption{My caption}
\label{tab:mytab}
\begin{tabular}{|p{1.0cm}|c|}
\hline
\textbf{Number} & \textbf{Images} \\ \hline
Nr. 1 & \raisebox{-.5\totalheight}{\includegraphics[width=0.8\textwidth]{example-image}} \\ \hline
Nr. 2 & \raisebox{-.5\totalheight}{\includegraphics[width=0.8\textwidth]{example-grid-100x100pt}} \\ \hline
\end{tabular}
\end{table}
\end{document}

To get even more precise centering, the downward shift of the image should be compensated by 1/2 the height of a line of text, thus, the shift should proceed as \raisebox{-.5\dimexpr\totalheight-\ht\strutbox}{...}
.
Note I also widened the first column to 1.3cm
, so as to allow the word Number
to fit.
\documentclass{article}
\usepackage{graphicx}
\begin{document}
\begin{table}[htbp]
\centering
\caption{My caption}
\label{tab:mytab}
\begin{tabular}{|p{1.3cm}|c|}
\hline
\textbf{Number} & \textbf{Images} \\ \hline
Nr. 1 & \raisebox{-.5\dimexpr\totalheight-\ht\strutbox}{%
\includegraphics[width=0.8\textwidth]{example-image}} \\ \hline
Nr. 2 & \raisebox{-.5\dimexpr\totalheight-\ht\strutbox}{%
\includegraphics[width=0.8\textwidth]{example-grid-100x100pt}}\\ \hline
\end{tabular}
\end{table}
\end{document}

\raisebox{-.5\totalheight}{}