package graphicx and ifthen problem

You need an expandable test, which \ifthenelse isn't.

\documentclass{article}
\usepackage[final]{graphicx}
\usepackage{etoolbox}

\newtoggle{GraphicsDraft}
\togglefalse{GraphicsDraft}

\newcommand{\nograph}{%
  \iftoggle{GraphicsDraft}{true}{false}%
}

\begin{document}

\includegraphics[draft=\nograph]{example-image-a}

\toggletrue{GraphicsDraft}

\includegraphics[draft=\nograph]{example-image-b}

\end{document}