You can accomplish this by setting up Custom Taxonomies along with your Custom Post type. You also might need to do some renaming to your custom-post type vs. custom taxonomy to get things to work the way you would like. here's a quick example:
// PORTFOLIO TAXONOMY
register_taxonomy( 'portfolio',
array('images'), /* This is the name of your custom post type, I used "Images" */
array('hierarchical' => true, /* if this is true it acts like categories */
'labels' => array(
/* OPTIONS */
),
'show_ui' => true,
'query_var' => true,
)
);
This will create a new metabox in your custom post type's edit screen, you can create "Portfolio" taxonomies on the fly. and the links would looks something like this:
mysite.com/portfolio/new-portfolio-slug/
Here is the list of options available for your taxonomy http://codex.wordpress.org/Function_Reference/register_taxonomy and an independent article that may help you further http://someweblog.com/wordpress-custom-taxonomy-with-same-slug-as-custom-post-type/
Hope this helps.