\documentclass{article}
\usepackage{float}
\usepackage[caption = false]{subfig}
\usepackage[final]{graphicx}
\begin{document}

\section{Appendix figures}

\begin{figure}[H]
\subfloat[Random M and M]{\includegraphics[width = 3in]{./img/random_H_random_M}} ~
\subfloat[Random H, batch M]{\includegraphics[width = 3in]{./img/random_H_batch_M}}\\
\subfloat[Batch H, random M]{\includegraphics[width = 3in, height = 2in]{example-image}}~
\subfloat[Batch H and M]{\includegraphics[width = 3in]{./img/lakkarajus_H_batch_M}}
\caption{Figures with different configs.}
\label{some example}
\end{figure}

\begin{figure}[H]
\subfloat[Independent H and random M]{\includegraphics[width = 3in, height = 2in]{example-image}} ~
\subfloat[Independent H, batch M]{\includegraphics[width = 3in]{./img/independent_H_batch_M}}\\
\subfloat[Probabilistic H, random M]{\includegraphics[width = 3in]{./img/probabilistic_H_random_M}}~
\subfloat[Probabilistic H, batch M]{\includegraphics[width = 3in]{./img/probabilistic_H_batch_M}}\\
\subfloat[Probabilistic H, batch M, $\beta_Z=\gamma_Z=5$]{\includegraphics[width = 3in]{./img/probabilistic_H_batch_M_coef_betaZ_5}} ~
\subfloat[Probabilistic H, batch M, max$(r)=0.5$.]{\includegraphics[width = 3in]{./img/sl_rmax05}}
\caption{Figures with different configs. (cont.)}
\label{some example}
\end{figure}
\end{document}