IMO, I think you should go about the problem differently. Instead of messing with the IEEE style, why not superimpose the reference text/label on the figure?
I think you can find several ways to do this.
With overpic or with tikz to name the first found search results at stack exchange.
Then you can use the referencing command \cite{bib-key} just like you would normally. In the MWE below, after finding the correct coordinates you can turn off the grid by removing that from the overpic options. Also, since you might need a difference color for the reference you can change the color of the text as well.
\documentclass[conference]{IEEEtran}
\usepackage{overpic,color}
\begin{document}
With grid for adjusting the reference in white.
\begin{figure}[htpb]
\begin{overpic}[width=0.9\textwidth,grid]{some_image.jpg}
\put(3,28){\huge\color{white}~\cite{IEEEhowto:kopka}}
\end{overpic}
\caption{Reference in white.}
\end{figure}
\begin{thebibliography}{1}
\bibitem{IEEEhowto:kopka}
H.~Kopka and P.~W. Daly, \emph{A Guide to \LaTeX}, 3rd~ed.\hskip 1em plus
0.5em minus 0.4em\relax Harlow, England: Addison-Wesley, 1999.
\end{thebibliography}
\end{document}