Hi,
in 2011 and 2012 we contacted the winners with a private wikimail. I recently had the honor to do this for an other competition (WikiLovesPublicArt in Austria) and had to find out that there is a new spam-protection on commons that only allows to send 5 mails and than my user was blocked to send wikimails for hours. Since (at least in Austria) we also had prices for more than the top 10, it would take weeks to contact them all. I think we need an alternative way to contact the winners or a way to send more wikimails - which in my eyes is the best way to contact users that are new to commons and don't look at their discussion page...
Best Ruben