Grâce aux remarques de @touhami, et l'option `valign=t` de l'extension [adjustbox](https://www.ctan.org/pkg/adjustbox), qui aligne précisément la figure avec le haut de la ligne supérieure, j'ai corrigé mon ECM, qui fonctionne désormais :
\documentclass[a4paper, 12 pt, showframe, french]{scrartcl}
\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[light, largesmallcaps]{kpfonts}
\usepackage{mwe}
\usepackage{geometry}
\usepackage{lipsum}
\usepackage{microtype}
\usepackage{grid-system}
\usepackage[Export]{adjustbox}
\usepackage{babel}
\begin{document}
\begin{row}{4}{2}
\begin{cell}{3}
\lipsum[1]
\end{cell}
\begin{cell}{1}
{\centering \includegraphics[keepaspectratio = true, width = \textwidth, valign=t]{example-image-a}}
\end{cell}
\end{row}\\
\begin{row}{1}{1}
\begin{cell}{1}
\raisebox{-\totalheight}{\centering \includegraphics[keepaspectratio = true, width = \textwidth]{example-image-a}}
\end{cell}
\end{row}\\
\begin{row}{1}{1}
\begin{cell}{1}
\raisebox{-\totalheight}{\centering \includegraphics[keepaspectratio = true, width = \textwidth]{example-image-b}}
\end{cell}
\end{row}
\end{document}
