Getting activity theme to use imagecache profiles

Hello, I'm trying to theme my activity output with imagecache profiles. The activity theme looks like this:

function theme_activity_user_picture($account) {
  return theme('user_picture', $account);
}

I did something like this:

 function myzensubtheme_activity_user_picture($account) {
return theme('imagecache', 'tinyavatar', 'user_picture', $account);
}

That doesn't quite do it, it links to tinyavatar folder but not the pictures/ folder inside. Any help would be greatly appreciated. Thanks.