\documentclass{book}
\usepackage{graphicx}
\usepackage{caption}
\DeclareCaptionTextFormat{myformat}{-- #1 --}
\captionsetup[figure]{textformat=myformat,labelformat=empty}
\begin{document}
\begin{figure}[htbp]
\centering
\includegraphics[width=.5\textwidth]{example-image-duck}
\caption{legende}
\caption{légende}
\end{figure}
\listoffigures
\end{document}
![alt text][1]
[1]: https://texnique.fr/osqa/upfiles/Screen_Shot_2019-02-12_at_21.01.45.pnghttps://texnique.fr/osqa/upfiles/Screen_Shot_2019-02-12_at_21.03.09.png
