[Tex/LaTex] center an image in a custom minipage

Question

Sorry for my broken language, I'm a Frenchy !

I have to create a page template for a personal project :
a box without margins in the 72% of the page, aligned to the top which contains a big image vertically and horizontally centered, and after it, in the same page, restart with normal text.
I try to compose a command :

   \newcommand\inspic[1]{
   \newpage
   \thispagestyle{empty}
   \noindent
   \begin{adjustwidth}{-1.8cm}{-1.8cm}
   \begin{minipage}[t]{\parwidth}
   \begin{minipage}[b][0.72\paperheight][b]{\paperwidth}
   \begin{center}
   \centerline{\includegraphics{#1}}
   \end{center}
   \end{minipage}
   \end{minipage}
   \end{adjustwidth}
   }

but I can not center the image vertically.
Here an image to explain it :

a minimal example :

\documentclass{report}
\usepackage[utf8]{report}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{graphicx}
\usepackage[french, english]{babel}
\usepackage{geometry}
\usepackage{blindtext}
\usepackage{changepage}

\geometry{
a4paper,
lmargin=1.8cm,
rmargin=1.8cm,
tmargin=2cm,
bmargin=3cm
}

\newcommand\inspic[1]{
\newpage
\thispagestyle{empty}
\noindent
\begin{adjustwidth}{-1.8cm}{-1.8cm}
\begin{minipage}[t]{\parwidth}
\begin{minipage}[b][0.72\paperheight][b]{\paperwidth}
\begin{center}
\centerline{\includegraphics{#1}}
\end{center}
\end{minipage}
\end{minipage}
\end{adjustwidth}
}

\begin{document}
\inspic{mybigpicture}
\section*{title}
some text here
\end{document}

SOLUTION :

\documentclass{report}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[french, english]{babel}
\usepackage{geometry}

\usepackage{lmodern}
\usepackage{graphicx}

\usepackage{lipsum}

\geometry{
  a4paper,
  lmargin=1.8cm,
  rmargin=1.8cm,
  tmargin=2cm,
  bmargin=3cm
}

\newcommand\inspic[1]{%
  \newpage
  \thispagestyle{empty}
  \noindent
  \makebox[\textwidth]{%
    \raisebox{2cm}[\dimexpr\height-2cm]{%
      \includegraphics[height=0.72\paperheight,keepaspectratio]{#1}%
    }%
  }\par
}

\begin{document}
\inspic{fuukeicool}
\section*{title}
\lipsum
\end{document}

Answer 1

Use a box \textwidth wide, which contains the picture raised by 2cm, but with its height trimmed by the same 2cm, so it will be placed at the very top of the page.

\documentclass{report}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[french, english]{babel}
\usepackage{geometry}

\usepackage{lmodern}
\usepackage{graphicx}

\usepackage{lipsum}

\geometry{
  a4paper,
  lmargin=1.8cm,
  rmargin=1.8cm,
  tmargin=2cm,
  bmargin=3cm
}

\newcommand\inspic[1]{%
  \newpage
  \thispagestyle{empty}
  \noindent
  \makebox[\textwidth]{%
    \raisebox{2cm}[\dimexpr\height-2cm]{%
      \includegraphics[height=0.72\paperheight,width=\paperwidth]{#1}%
    }%
  }\par
}

\begin{document}
\inspic{example-image-a.pdf}
\section*{title}
\lipsum
\end{document}

If the picture is not as wide and high as the reserved box, for instance it's higher than wide, there's no difference, just don't specify the width:

\documentclass{report}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[french, english]{babel}
\usepackage{geometry}

\usepackage{lmodern}
\usepackage{graphicx}

\usepackage{lipsum}

\geometry{
  a4paper,
  lmargin=1.8cm,
  rmargin=1.8cm,
  tmargin=2cm,
  bmargin=3cm
}

\newcommand\inspic[1]{%
  \newpage
  \thispagestyle{empty}
  \noindent
  \makebox[\textwidth]{%
    \raisebox{2cm}[\dimexpr\height-2cm]{%
      \includegraphics[height=0.72\paperheight]{#1}%
    }%
  }\par
}

\begin{document}
\inspic{example-image-9x16}
\section*{title}
\lipsum
\end{document}