\documentclass[11pt]{article}
%\usepackage{geometry}                % See geometry.pdf to learn the layout options. There are lots.
%\geometry{letterpaper}                   % ... or a4paper or a5paper or ... 
%%\geometry{landscape}                % Activate for for rotated page geometry
%%\usepackage[parfill]{parskip}    % Activate to begin paragraphs with an empty line rather than an indent
\usepackage{graphicx}
%\usepackage{amssymb}
%\usepackage{epstopdf}
%\DeclareGraphicsRule{.tif}{png}{.png}{`convert #1 `dirname #1`/`basename #1 .tif`.png}
%
%\title{Brief Article}
%\author{The Author}
%%\date{}                                           % Activate to display a given date or no date

\begin{document}
%\maketitle
%\section{}
%\subsection{}
\section{Als Original...}

%\begin{figure}[htbp]
\includegraphics{testbild.png}
\includegraphics{testbild.jpg}
\includegraphics{testbild.pdf}
\newpage
\section{Etwas kleiner...}
\includegraphics[width=0.7\textwidth]{testbild.png}
\includegraphics[width=0.3\textwidth]{testbild.jpg}
\includegraphics[width=0.5\textwidth]{testbild.pdf}

\newpage
\section{Bild zentrieren}
\begin{center}
\includegraphics[width=0.5\textwidth]{testbild.png}
\end{center}
\newpage
\section{Bildbeschriftung}
\begin{figure}[ht]
\begin{center}
\caption{Beschriftung oben}
\label{default}
\includegraphics[width=0.5\textwidth]{testbild.png}
\end{center}
\end{figure}

\begin{figure}[ht]
\begin{center}
\label{default}
\includegraphics[width=0.5\textwidth]{testbild.png}
\caption{Beschriftung unten}
\end{center}
\end{figure}


%\includegraphics[viewport= 0 100 100 200]{testbild.jpg}
%\includegraphics[width=0.5\textwidth, viewport= 0 300 20 20]{testbild.pdf}

%\begin{center}
%\caption{default}
%\label{default}
%\end{center}
%\end{figure}


\end{document}