I have achieved the following result:
With the code:
\begin{tikzpicture}[remember picture, overlay, transform shape]
\node [anchor=north, inner sep=0pt]
at (current page.north)
{
\tikz[remember picture,overlay] \node[opacity=0.3,inner sep=0pt]{\includegraphics[width=1.01\paperwidth]{images/header1}};
};
\draw (1, 1) node {Hello world};
\end{tikzpicture}
The problem is that I want the image to have full opacity. But if I do this, since the image is drawn OVER the text, it will disappear. I need the text to be drawn over the image instead.

backgroundlayer (seebackgroundstikzlibrary) – Ignasi Apr 25 '18 at 06:43tikzposter? So you want something like https://tex.stackexchange.com/questions/121000/adding-a-background-image-with-tikz-poster/309888#309888? – Torbjørn T. Jul 13 '18 at 14:48