Improve inclusion of image in preface

This includes not calling it a "figure" when no figure float is used.

chapters/preface.tex

@@ -4,9 +4,10 @@ \chapter*{Preface}\label{preface}
 Modelica is a freely available, object-oriented language for modeling of
 large, complex, and heterogeneous physical systems. From a user's point
 of view, models are described by schematics, also called object
-diagrams. Examples are shown in the next figure:
-
-\includegraphics[width=5.92986in,height=3.59167in]{diagram_examples.png}
+diagrams. Examples are shown below:
+\begin{center}
+\includegraphics[width=0.95\textwidth]{diagram_examples}
+\end{center}
 
 A schematic consists of connected components, like a resistor, or a
 hydraulic cylinder. A component has \emph{connectors} (often also called