Display only a single figure when in draft mode

Just set draft=false for the picture you want to be shown anyway:

\documentclass[draft]{article}

\usepackage{graphicx}

\begin{document}

\includegraphics[width=50pt]{example-image-a}

\includegraphics[draft=false,width=50pt]{example-image-b}

\includegraphics[width=50pt]{example-image-c}

\end{document}

Create a macro \includegraphicsfinal (say) which will always be set in final mode, regardless of whether your document is set in final or draft mode:

\documentclass[draft]{article}

\usepackage{graphicx}

\makeatletter
\newcommand{\includegraphicsfinal}[2][]{{%
  \Gin@draftfalse% Turn draft mode off
  \includegraphics[#1]{#2}}% Include graphic
\makeatother

\begin{document}

\includegraphics[width=50pt]{example-image-a}

\includegraphicsfinal[width=50pt]{example-image-b}

\includegraphics[width=50pt]{example-image-c}

\end{document}

Note the grouping inside \includegraphicsfinal which provides a localized scope for the change to \ifGin@draft.