To create an icon you can use

AbstractImagePrototype icon = IconHelper.create("path/to/png");

Within a TreeGrid it is possible to use a ModelIconProvider

TreeGrid<ModelData> grid = new TreeGrid<ModelData>(store, columnModel);
grid.setIconProvider(new MyIconProvider());

The provider can return an icon for different situations (like expanded or collapsed nodes, some special node types).

public class MyIconProvider implements ModelIconProvider<ModelData> {

   private final TreeGrid<ModelData> grid;

   public MyIconProvider(TreeGrid<ModelData> grid) {
      this.grid = grid;
   }

   public AbstractImagePrototype getIcon(ModelData model) {
      if (model instanceof Something) {
         return IconHelper.create("img/something.png");
      }
      
      if (grid.isExpanded(model)) {
         return IconHelper.create("img/folder-open.png");
      } else {
         return IconHelper.create("img/folder-close.png");
      }
   }
}