\documentclass{article}
\usepackage{background}
\usepackage{tikz}
\usepackage{pgfplots}
\usepackage{lipsum}
\usepackage{graphicx}

\backgroundsetup{
scale=1.5,
angle=0,
firstpage=true,
opacity=0.1,
contents={
\begin{tikzpicture}
\node at ([xshift=0pt, yshift=0pt] current page.center){\includegraphics{example-image-duck}};
\end{tikzpicture}
}}
\begin{document}
\lipsum[1-5]
\end{document}

\documentclass{article}
\usepackage{background}
\usepackage{tikz}
\usepackage{lipsum}

\backgroundsetup{
scale=1.5,
angle=0,
firstpage=true,
opacity=0.1,
contents={
\begin{tikzpicture}
\node at ([xshift=0pt, yshift=0pt] current page.center){\includegraphics{example-image-duck}};
\end{tikzpicture}
}}
\begin{document}
\lipsum[1-5]
\end{document}

Edit: For me it seem that there is a bug in the background package. In the documentation it says:

firstpage= This boolean option allows the user to specify, setting the value true, that the background material must be displayed only in the first page of the document. The deafulat value is false.

What really happens is that the material is placed on all pages independent of this option. -and It seems that the material is placed twice on the first page, if the option is true. -making the first page less opaque.

In your case, you can just keep the option with the default false like this:

\documentclass{article}
\usepackage{background}
\usepackage{lipsum}
\backgroundsetup{
scale=1.5,
angle=0,
contents={\includegraphics{example-image-duck}}
}
\begin{document}
\lipsum[1-150]
\end{document}