How to insert "source" below image

\documentclass{article}
\usepackage{graphicx}
\usepackage[x11names]{xcolor}

\usepackage{copyrightbox}
%%%% figure-copyright
    \usepackage{url}

\begin{document}
    \begin{figure}[!h]
\copyrightbox[b]{\includegraphics[width=0.6\linewidth]{example-image-b}}%
           {Source: \url{http://tex.stackexchange.com/}}

    \end{figure}
\end{document}

edit: or with stackengine

\documentclass{article}
\usepackage{graphicx}

\usepackage{stackengine}
\usepackage{url}

\begin{document}
    \begin{figure}[hb]
    \centering
\def\stackalignment{r}
\stackunder{\includegraphics[width=\linewidth]{example-image-b}}%
           {\scriptsize%
            Source: \url{http://tex.stackexchange.com/}}
\caption{Image with copyright}
    \end{figure}
\end{document}

Set your image in a tabular that has a single right-aligned column:

\documentclass{article}

\usepackage{graphicx}

\newcommand{\imagesource}[1]{{\footnotesize Source: #1}}

\begin{document}

\begin{figure}
  \centering
  \begin{tabular}{ @{} r @{} }
    \includegraphics[width = .5\linewidth]{example-image} \\
    \imagesource{Image A}
  \end{tabular}
\end{figure}

\end{document}