Clicking the Manage Icon button will open the Icon Manage for Entity icons.
In the Icon Manager, the built-in icons are categorized and can be browsed through or searched for using the search input field at the top of the window.
It is also possible to add new icons that can be used for new Entities that are created. To add a new Entity icon, click the plus (+) button in the top left-hand corner of the Icon Manager window. This will open another window where the image file for the new icon can be chosen:
Once the image file has been chosen, the category for the icon must be selected:
Clicking OK will add the new Entity icon to your Maltego client.