Centering an image vertically

\centerline takes an argument so in

\centerline\noindent\makebox[\textwidth]{\includegraphics[width=\paperwidth]{MyImage.jpg}

the argument is just \noindent

so it is equivalent to

\centerline{\noindent}%
\makebox[\textwidth]{\includegraphics[width=\paperwidth]{MyImage.jpg}}

so you get one blank white line from the \centerline then an overfull box with the image.

possibly you want

\vspace*{\fill}
\noindent
\hspace*{-\oddsidemargin}%
\makebox[0pt][l]{\includegraphics[width=\paperwidth]{MyImage.jpg}}
\vspace*{\fill}

I am assuming here you did intend your image to be as wide as the paper not just as wide as the text block, so covering both margins.

\documentclass[demo]{article}
\usepackage[a6paper,margin=2cm]{geometry}
\usepackage{graphicx}
\begin{document}
\null\vfill
\centering\includegraphics{xyz}
\vfill\null
\end{document}

How to prove?

\documentclass[pstricks,border=12pt]{standalone}

\usepackage{filecontents}
\begin{filecontents*}{foo.tex}
\documentclass[demo]{article}
\usepackage[a6paper,margin=2cm]{geometry}
\usepackage{graphicx}
\begin{document}
\null\vfill
\centering\includegraphics{xyz}
\vfill\null
\end{document}
\end{filecontents*}

\immediate\write18{pdflatex foo}
\immediate\write18{pdftops -f 1 -l 1 -eps foo.pdf}


\usepackage{graphicx}
\newsavebox\IBox
\savebox\IBox{\includegraphics{foo}}

\psset
{
    xunit=0.1\wd\IBox,
    yunit=0.1\ht\IBox,
}

\begin{document}
\begin{pspicture}[showgrid=top](10,10)
    \rput[bl](0,0){\usebox\IBox}
\end{pspicture}
\end{document}

The image can easily be centered horizontally and vertically via package pdfpages. It also enlarges the image as much as possible to fit into the paper area without distorting the image.

Example:

\documentclass[11pt,a4paper,BCOR=10mm,DIV=11]{scrbook}

\usepackage{pdfpages}

\begin{document}
  \includepdf{MyImage.jpg}
\end{document}

Option BCOR is not taken into account, when placing the image. Some white space can be added to the left or right of the image via option trim:

\includepdf[trim=-5mm 0 -8mm 0]{MyImage.jpg}

Would add a white margin of 5mm to the left and 8mm to the right of the unscaled image.