Upload some of the best resulting pictures as examples of a novel kind of online collaboration in appropriate forums to pique curiosity. Do some themed collaborations around upcoming holidays/events/controversies. (Superbowl, Chinese New Year, Valentine's Day, etc. – each output might carry further, and fit discussion in more places, for a few key days.)
Try to find fans/discussions of the precursor ('tiles') thing and mention Tzigla to them. Find discussions about collaborative art and make sure you're appropriately mentioned in context.
Make sure you have a Facebook fan page so any enthusiastic new users might inform larger networks that way. (EDIT: I see you've already done this.) You might even want to try some small Facebook ad experiments, targeted on people talking about 'art', 'painting', 'doodling', 'drawing', 'exquisite corpse', 'surrealism', etc., but otherwise as different from your existing users as possible (ages/regions/etc). At a very small price – maybe just dozens of dollars – that might find other totally new clusters of people as passionate as your founding userbase.
Good luck!