documentclass{article}

% for example images
usepackage{mwe}

% xparse is no necessary but allows a more flexible interface design
usepackage{xparse}
usepackage{graphics}

makeatletter
NewDocumentCommandplacefig{m O{} m m}{%
    begin{figure}%
        centering
        includegraphics[#2]{#3}%
        caption{#4}%
        label{#1}%
    end{figure}%
}
NewDocumentCommanddeffig{m O{} m m}{
    expandaftergdefcsname fig@r@#1endcsname{%
        placefig{#1}[#2]{#3}{#4}%
        expandafterglobalexpandafterletcsname fig@r@#1endcsnamerelax
    }%
}
letold@refref
renewcommand*ref[1]{%
    old@ref{#1}%
    ifcsname fig@r@#1endcsnamecsname fig@r@#1endcsnamefi
}
makeatother

% define the (possible) figures
deffig{fig:ex-a}[width=.5textwidth]{example-image-a}{This is an example image.}
deffig{fig:ex-b}{example-image-b}{This is another example image.}

begin{document}

This is the first reference to fig.~ref{fig:ex-a} in the document.

Here, we reference fig.~ref{fig:ex-a} again.

end{document}