I guess that this is a kind of private joke that only portuguese people are able to understand. In Portugal we have the expression «lançamento da primeira pedra» it meas literally something like "throwing the first rock" and it is more or less a solemn act on the beginning of every new construction, where someone "important" like a president or something, "make the honors".
So. This is a poster illustrative of the "thrown of the first rock" of our Tua dam, by our prime minister José Socrates...at least, how i wish it was.