\documentclass{a0poster}

\usepackage{graphicx,lipsum}

\begin{document}

\sloppy% Just for this example
\lipsum[1-4]

\begin{figure}[h!]
  \includegraphics[width=\linewidth,height=4\baselineskip]{example-image}
\end{figure}

\lipsum[5-10]

\end{document}