How To create a Portfolio item
First of all you need the Multi Portfolio plugin installed. You can install this plugin as explained here:
Once you have that plugin installed, a new menu is created called Portfolio, go there and click on Add New item:
Add you Title and Content as normal and select your Featured Image here:
Also you will see a section called Portfolio Item Options, with these options
Here you can choose if you want a normal size item or a big one. Example:
This option is available on Portfolio Masonry only, so you can make a particular item bigger than the rest.
All the portfolio categories you select here will be used for the portfolio filter: