I was using default pictures for the users that doesn't upload any, but they don't get displayed by the site user list moduleof drupal.

Actualy What the template is doing is creating a URL based on what is in the database. If you look at the picture column of your users table, that is the value that is being substituted for @picture.

In my case the problem was the default template which was loading : , but @_base was empty, so i changed the template to and it works well for me.

the user list gets displayed along with the pictures.