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\NeedsTeXFormat{LaTeX2e}
\documentclass[portrait]{sciposter}

\usepackage[ngerman]{babel}
\usepackage[ansinew]{inputenc}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{multicol}
\usepackage{graphicx}


\begin{document}

  \begin{figure}[h!]
    \begin{center}
    \includegraphics[width=20cm]{./test.png}
    \end{center}
    \caption{Dies ist ein Testbild}
    \label{pic:test}
  \end{figure}

\end{document}

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\usepackage[hang,xmall,bf]{caption}

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\addto\captionsngerman{
  \renewcommand{\figurename}{Abbildung}
} 

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\renewcommand{\capstart}{\bf Figure \arabic{figure}: }

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\renewcommand{\capstart}{%
    \figcapstartstyle{\figurename\nobreakspace\thefigure:}%
}