##Avec `pdfpages`:
\documentclass[a4paper, 12pt, syldavian, french, BCOR=0pt, DIV=21]{scrartcl}
\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[light, largesmallcaps]{kpfonts}
\usepackage[final]{pdfpages}
\usepackage{babel}
\begin{document}
Facture numéro, etc.
\newpage
\section{Pièces justificatives}
\includepdf[picturecommand*={\put(100,100){\includegraphics[width=3cm]{example-image}}}]{example-image-a4}%
\includepdf[picturecommand*={\put(100,100){\includegraphics[width=3cm]{example-image-duck}}}]{example-image-a4}%
\end{document}
##Avec `tikz`:
\documentclass[a4paper, 12pt, syldavian, french, BCOR=0pt, DIV=21]{scrartcl}
\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[light, largesmallcaps]{kpfonts}
\usepackage{babel}
\usepackage{tikz}
\begin{document}
Facture numéro, etc.
\newpage
\section{Pièces justificatives}
\newpage
\thispagestyle{empty}
\begin{tikzpicture}[overlay, remember picture]
\node at (current page.center) {\includegraphics[width=\paperwidth]{example-image-a4}};
\node at (8,-22) {\includegraphics[width=3cm]{example-image}};
{\includegraphics[width=3cm]{example-image-duck}};
\end{tikzpicture}
\end{document}
![alt text][1]
[1]: https://texnique.fr/osqa/upfiles/Screen_Shot_2019-03-13_at_23.01.02.pnghttps://texnique.fr/osqa/upfiles/Screen_Shot_2019-07-05_at_14.06.03.png
