The photo is of someone shooting a 100lb + warbow..... something most archers would have great difficulty doing using a "normal" stance. The most somfortable way of drawing a heavy bow (and remember true medieval longbows get harder to hold the more you draw) is to really put your back and shoulder muscles into it. The wider the stance the easier this is.
More pictures of same person and aforementioned Mark Stretton shooting are on this link :
http://www.bickerstaffebows.co.uk/european_tour.htm