Placeholder for figure/includegraphics

It's 2018 now, and this has become very easy.

Simply:

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

The mwe package needs to be installed for this (but it doesn't need to be included in the preamble). If using MiKTex e.g., it will offer to install this package when the document is built.

There are other placeholder images available as well.

Example document from another StackExchange answer:

\documentclass[12pt,a4paper]{article}
\usepackage{graphicx}

\begin{document}

\noindent\includegraphics[width=3cm]{example-image-a}\qquad
\includegraphics[width=3cm]{example-image-golden}\qquad
\includegraphics[width=3cm]{example-grid-100x100pt}

\noindent\includegraphics[height=5cm]{example-image-b} 

\noindent\includegraphics[scale=0.5]{example-image-c} 

\noindent\includegraphics[width=3cm]{example-image} 

\end{document}

todonotes provides the \missingfigure command, precisely designed for this case

\documentclass{article}
\usepackage{todonotes}

\begin{document}

\missingfigure[figwidth=6cm]{Testing a long text string}

\end{document}

If you need individual figures to be omitted rather than all of them, you first copy a dummy figure (called foo here) to your figures path, then use as:

\includegraphics[draft]{foo}

It will display something like:

It will also save ink, and you can use \includegraphics syntax as it is, you just need to replace foo and remove draft from command.