I want the same thing as those guys:
But with one minor difference: I don't like the idea to write manually (a) instead of using \subcaption{}. Why? Because by arranging the figures differently later on, I could forget to change the labels accordingly.
Now I did like the idea of using the stackengine with its \stackinset-command. However, it does not accept \subcaption{} as an argument.
So my minimum non-working example is as follows (the non-working part is the \subcaption{} command at its current location):
\documentclass{article}
\usepackage{graphicx}
\usepackage{caption,subcaption}
\usepackage{stackengine}
\begin{document}
\begin{figure}
\begin{subfigure}{.96\textwidth}
\stackinset{l}{1in}{b}{1in}{\subcaption{}}{\includegraphics[width=5in]{example-image-a}}
\label{fig:test}
\end{subfigure}
\end{figure}
\end{document}
using \subref within the \stackinsetas another answer! – Gus May 30 '21 at 19:17