Your problem will be solved by setting the featured image for each portfolio item as shown below –
Pls have a look at your site and you should see your portfolio items now. Pls do not use the shortcode on the page templates which start with name ‘Portfolio ….’. Thanks
BTW – the sample data import should have set the featured image for each of the portfolio item imported. May be the images were deleted or the import was interrupted ( you should have the import attachments option checked when importing).