Here is the LaTeX code to inserto 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]{.6linewidth}vspace{0pt}
  includegraphics[width=linewidth]{image1.eps}
  label{fig:image1}
 end{minipage}
 hfill
 begin{minipage}[t]{.4linewidth}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*}