Voici une solution qui demande deux passes.
![alt text][1]
\documentclass[a4paper, 12pt]{scrartcl}
\usepackage{kpfonts}
\usepackage[T1]{fontenc}
\usepackage[savepos]{zref}
\usepackage[Export]{adjustbox}
\usepackage{mwe}
\newcounter{mtimg}[page]
\makeatletter
\newenvironment{mtfigure}
{\par\noindent
\begin{minipage}[t]{\textwidth}%
\def\@captype{figure}
\centering
\ifnum\value{mtimg}=0
\stepcounter{mtimg}%
\ifodd\value{page}%
\zsaveposy{pg\arabic{page}}%
\else
\@ifundefined{Z@R@pg\the\numexpr\value{page}-1}\relax{\vskip\dimexpr\paperheight-\zposy{pg\the\numexpr\value{page}-1}sp-\topmargin-\headheight-\headsep-\topskip-1in\relax}
\fi}
\fi\fi}
{\end{minipage}\par}
\makeatother
\begin{document}
\section{Première partie}
\subsection{Première sous-partie}
\begin{mtfigure}
\includegraphics[width=\textwidth,valign=T]{example-image-a}
\captionof{figure}{Une première illustration}
\label{fig:1a}
\end{mtfigure}
\newpage
\begin{mtfigure}
\includegraphics[width=\textwidth, height=8cm,valign=T]{example-image}
\captionof{figure}{Une deuxième illustration}
\label{fig:1b}
\end{mtfigure}
\newpage
\begin{mtfigure}
\includegraphics[width=\textwidth,valign=T]{example-image-a}
\captionof{figure}{Une première illustration}
\label{fig:2a}
\end{mtfigure}
\newpage
\begin{mtfigure}
\includegraphics[width=\textwidth, height=8cm,valign=T]{example-image}
\captionof{figure}{Une deuxième illustration}
\label{fig:2b}
\end{mtfigure}
\newpage
\subsection{Première sous-partie}
\begin{mtfigure}
\includegraphics[width=\textwidth,valign=T]{example-image-a}
\captionof{figure}{Une première illustration}
\label{fig:3a}
\end{mtfigure}
\newpage
\begin{mtfigure}
\includegraphics[width=\textwidth, height=8cm,valign=T]{example-image}
\captionof{figure}{Une deuxième illustration}
\label{fig:3b}
\end{mtfigure}
\end{document}\end{document}
[1]: https://texnique.fr/osqa/upfiles/22.png
