Here is the LaTeX code to insert two images of different dimensions and a caption in this format:

The code uses minipages (as usual!) to solve this problem.
\begin{figure*}
\begin{minipage}[t]{.6\linewidth}\vspace{0pt}
\includegraphics[width=\linewidth]{image1.eps}
\label{fig:image1}
\end{minipage}
\hfill
\begin{minipage}[t]{.4\linewidth}\vspace{0pt}\raggedright
\begin{minipage}[t]{\linewidth}\vspace{0pt}\raggedright
\includegraphics[width=\linewidth]{image2.eps}
\label{fig:image2}
\end{minipage}
\begin{minipage}[t]{\linewidth}\vspace{20pt}\raggedright
\caption{Caption [...]}
\label{fig:spidx}
\end{minipage}
\end{minipage}
\end{figure*}