[Tex/LaTex] remove tikzpicture border

Question

I am trying to load a picture as a whole pdf page, so I use below code to load the image as a node and use adjust box package to enlarge it to whole page:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{backgrounds}
\usepackage{graphicx}
\usepackage[a4paper,margin=0pt]{geometry}
\usepackage{adjustbox}

\begin{document}
%https://upload.wikimedia.org/wikipedia/commons/1/16/Mycena_interrupta.jpg
\def\imgfile{Mycena_interrupta.jpg}
\begin{adjustbox}{width=\paperwidth,height=\paperheight}
  \begin{tikzpicture}[background rectangle/.style={fill=red!40},show background rectangle]
    \node (N) { \includegraphics{\imgfile}};
  \end{tikzpicture}
\end{adjustbox}
\clearpage
\end{document}

But it seems tikzpicture has a border,just as the red!40 color indicated:

Answer 1

Following my comment, this is my solution:

\documentclass{article}
\usepackage{tikz}
\begin{document}
\pagestyle{empty}
\begin{tikzpicture}[remember picture,overlay]
\node at (current page.south west) [above right,inner sep=0,outer sep=0] 
    {\includegraphics[width=\paperwidth,height=\paperheight]{Mycena_interrupta}};
\end{tikzpicture}
\end{document}