Page background in ShareLaTeX

Question

How can I add an image as background in ShareLaTeX?

Answer 1

...just like you would in any LaTeX environment. Here's an option using eso-pic:

\documentclass{article}

\usepackage{eso-pic,graphicx,lipsum}

\begin{document}

\AddToShipoutPictureBG*{
  \AtPageLowerLeft{%
    \includegraphics[width = \paperwidth, height = \paperheight]{example-image}%
  }%
}

\lipsum% Your document

\end{document}

The starred version places content in the background on the current page only.