I have set this page to pull in posts for a specific category ID but by default it will grab all.

By default, the posts being pulled in need to have a post thumbnail set but you can set a custom field called thumbnail and thumbnail_link on the posts being pulled in if you want to have full control over the thumbnail and where it links to.

You can also add images/links/titles in manually without having to create additional posts, please see the sub-portfolio in the menu. (This text above the portfolio items is totally optional)