update images to be in pdf form

chapters/chap02/chap02.tex

@@ -254,7 +254,7 @@ emerged.\\
     \put(0, 1.75){
       \begin{subfigure}{0.4\textwidth}
         \centering
-        \includegraphics[width=\textwidth]{reference_params_synth.png}
+        \includegraphics[width=\textwidth]{reference_params_synth.pdf}
         \caption{$\alpha=0.35$, $\beta=0.5$}
         \label{fig:synth_norm}
       \end{subfigure}
@@ -263,7 +263,7 @@ emerged.\\
     \put(5.5, 5){
       \begin{subfigure}{0.3\textwidth}
         \centering
-        \includegraphics[width=\textwidth]{low_beta_synth.png}
+        \includegraphics[width=\textwidth]{low_beta_synth.pdf}
         \caption{$\alpha=0.25$, $\beta=0.5$}
         \label{fig:synth_low_beta}
       \end{subfigure}
@@ -272,7 +272,7 @@ emerged.\\
     \put(9.5, 5){
       \begin{subfigure}{0.3\textwidth}
         \centering
-        \includegraphics[width=\textwidth]{high_beta_synth.png}
+        \includegraphics[width=\textwidth]{high_beta_synth.pdf}
         \caption{$\alpha=0.45$, $\beta=0.5$}
         \label{fig:synth_high_beta}
       \end{subfigure}
@@ -281,7 +281,7 @@ emerged.\\
     \put(5.5, 0){
       \begin{subfigure}{0.3\textwidth}
         \centering
-        \includegraphics[width=\textwidth]{low_alpha_synth.png}
+        \includegraphics[width=\textwidth]{low_alpha_synth.pdf}
         \caption{$\alpha=0.35$, $\beta=0.4$}
         \label{fig:synth_low_alpha}
       \end{subfigure}
@@ -290,7 +290,7 @@ emerged.\\
     \put(9.5, 0){
       \begin{subfigure}{0.3\textwidth}
         \centering
-        \includegraphics[width=\textwidth]{high_alpha_synth.png}
+        \includegraphics[width=\textwidth]{high_alpha_synth.pdf}
         \caption{$\alpha=0.35$, $\beta=0.6$}
         \label{fig:synth_high_alpha}
       \end{subfigure}