import java.io.File;    
    import java.net.URL;  
    import javax.swing.UIManager;  
    import javax.swing.JFrame;    
    import javax.swing.JTree;    
    import javax.swing.JScrollPane;    
    import javax.swing.tree.DefaultMutableTreeNode;   
    import javax.swing.tree.TreePath;   
    import javax.swing.JOptionPane;  
    import javax.swing.Icon;  
    import javax.swing.ImageIcon;  
     
       
    class Program extends JFrame{    
       
        public Program(){    
          
            URL resource = getClass().getResource("NodeImage.jpg");  
            Icon icon = new ImageIcon(resource);  
     
            UIManager.put("Tree.closedIcon", icon);  
            UIManager.put("Tree.openIcon", icon);  
            UIManager.put("Tree.leafIcon", icon);  
          
            DefaultMutableTreeNode root = new DefaultMutableTreeNode("Fruits");    
              
            DefaultMutableTreeNode node1 = new DefaultMutableTreeNode("Apples");  
            DefaultMutableTreeNode node2 = new DefaultMutableTreeNode("Grapes");  
            DefaultMutableTreeNode node3 = new DefaultMutableTreeNode("Oranges");  
            DefaultMutableTreeNode node4 = new DefaultMutableTreeNode("Bananas");  
              
            root.add(node1);  
            root.add(node2);  
            root.add(node3);  
            root.add(node4);  
          
            JTree tree = new JTree(root);      
            JScrollPane treeView = new JScrollPane(tree);    
                
            this.getContentPane().add(treeView);    
        }    
             
        public static void main(String[] args){      
            Program p = new Program();    
            p.setSize(300,400);    
            p.setVisible(true);    
        }    
    }

http://www.hostprojects.net/snippets/java/141/add-icon-to-jtree-nodes-using-the-uimanager

This code snippet uses the UIManager to add icons to JTree nodes.

Add Icon To JTree Nodes Using The UIManager

Java