Using \ifthenelse inside an option

Use a real optional argument

\newcommand{\introimg}[4][0.9]{
\begin{figure}
    \centering
    \includegraphics[width=#1\textwidth]{Figures/#2}
    \caption{#4}
    \label{#3}
\end{figure}
}

to be called like

\introimg{filename}{label}{caption}

or

\introimg[0.5]{filename}{label}{caption}

You don't need to use a conditional. The optional argument of a command is #1, so you should define your command in the following way:

\documentclass{article}
\usepackage[demo]{graphicx}
\newcommand{\introimg}[4][.9]{
\begin{figure}
    \centering
    \includegraphics[width=#1\textwidth]{Figures/#2}
    \caption{#3}
    \label{#4}
\end{figure}
}
\begin{document}
\introimg{Foo}{Caption}{Fig:foo}
\introimg[.7]{Foo}{Caption}{Fig:foo}
\end{document}