\setlength{\unitlength}{1bp}
\begin{picture}(540,180)
\small
\put(0,0){\includegraphics{pp}}
\put(165,20){\makebox(0,0){$\alpha(t)$}}
\put(20,160){\makebox(0,0){$\beta(t)$}}
\put(345,20){\makebox(0,0){$\alpha(t)$}}
\put(200,160){\makebox(0,0){$\gamma(t)$}}
\put(525,20){\makebox(0,0){$\beta(t)$}}
\put(380,160){\makebox(0,0){$\gamma(t)$}}
\end{picture}