Piwik: tracking actionsPiwik is an open-source web analysis tool, a bit like Google Analytics, but it's supposed to be installed on your own server. After being nagged by Robe I decided to test it on a low-traffic site (actually Robe also wrote all code to make this happen).
So, piwik was just ever seeing the initial page load, but not when a user clicked on such a picture link. Therefore the stats were mostly useless.
But: piwik can actually track custom supplied actions, and you could call the piwik tracking code