Thumbnail(Featured) Image Size Options not available

Hi,
the three featured image sizes that are available – “slider”, “featured”, and “small featured” – are not showing up as options.

I opened up the function.php file and found this text:

* Note that this function is hooked into the after_setup_theme hook, which runs
 * before the init hook. The init hook is too late for some features, such as indicating
 * support post thumbnails.

Is there any way to get these options back?