Thanks, crio, glad you like it!
Unfortunately, PhotoSmash doesn't do this, but you can totally do it with jQuery.
The question would be which element you'd want to add the class to (the li, div, img, etc)? Then, it's just a matter of running it through .each().
Here's some basic code that should work for adding it to the img (not tested):
jQuery(document).ready(function() {
var i = 0;
jQuery("img.ps_images").each(function() {
if (i % 3 == 0) {
jQuery(this).addClass('my-new-class');
}
});
}
Here's a StackOverflow article on the problem too: http://stackoverflow.com/questions/1260277/addclass-every-nth
Cheers,
Byron