\begin{figure}[h]
\centering
\makebox
{
    \setlength{\unitlength}{1cm}
    \begin{picture}(6,3)
    \put(0,0){\circle*{0.2}}
    \put(0,3){\circle*{0.2}}
    \put(6,3){\circle*{0.2}}
    \put(6,0){\circle*{0.2}}

    \thicklines
    \put(1,1){\line(1,0){4}}
    \put(5,1){\line(0,1){1}}
    \put(1,1){\line(4,1){4}}

    \put(1,1){$C$}
    \put(5.05,1){$A$}
    \put(5.05,2){$B$}

    \end{picture}
}
\caption{\label{fig:rysunek2} Obrazek 6 cm na 3 cm.}
\end{figure}