If you want to use custom icons (images), override the following CSS classes for their respective uses, on the treetable:
ui-icon
: to customise the expanded-row state icon (the triangle).ui-icon { width: 16px; height: 16px; background-image: url("/your-image-here"); }
ui-icon-triangle-1-e ui-c
: to customize the collapsed-row state icon
If you want to use more modern fontAwesome 'icons', see Change icon from jQuery UI to FontAwesome in PrimeFaces