When I am putting two round images side by side, their shapes change to oval! here is how I do it:
\begin{figure}[t!]
%
\begin{subfigure}[t]{0.45\linewidth}
\centering
\includegraphics[width=4.2cm, height=5.0cm]{Probabilistic_Vs_Random.png}
% \vspace{1.5cm}
\caption{probabilistic-based player starts the game}\medskip
\label{fig:2a}
\end{subfigure}\hfill
\begin{subfigure}[t]{0.45\linewidth}
\centering
\includegraphics[width=4.2cm, height=5.0cm]{Random_Vs_Probabilistic.png}
% \vspace{1.5cm}\hfill
\caption{random player starts the game}\medskip
\label{fig:2b}
\end{subfigure}\hfill
\begin{subfigure}[t]{0.45\linewidth}
\centering
\includegraphics[width=4.2cm, height=5.0cm]{Probabilistic_Vs_Probabilistic.png}
% \vspace{1.5cm}
\caption{probabilistic vs. probabilistic}\medskip
\label{fig:2c}
\end{subfigure}
\caption{Performance of random and probabilistic-based players based on 1000 plays} \label{fig:2}
\end{figure}
I guess the problem is the size of my plots and this line
\includegraphics[width=4.2cm, height=5.0cm
but I don't know how to fix. here is the snapshot

width=orheight=options to maintain the aspect ratio. – Paul Gessler Aug 15 '14 at 19:10scale= …option. – Bernard Aug 15 '14 at 19:11keepaspectratio. – Torbjørn T. Aug 15 '14 at 19:20