how to set image to Java swing background and buttons

Hello I'm trying to change one thing in my code and thats including an image in my background instead of simply a color. I put in the url inside of my code but it gives an error stating that I cant convert a color to string. what am I doing wrong here?

package blackjack;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

 /**
 *
  * @author tacomeat
 */
public class BlackjackGUI extends JFrame {

public static void main( String [] args ) {
    JFrame frame = new BlackjackGUI();
    frame.getContentPane().setBackground( FOREST_GREEN );        
    frame.setSize( 580, 480 );

    frame.show();
}

private BlackjackDealer dealer;
private GUIPlayer       human;
private final JPanel players = new JPanel( new GridLayout( 0, 1 ) );
private static PlayerView Dealerview;

private static final Color FOREST_GREEN = new Color( 35, 142, 35 );


public BlackjackGUI() {  
    setUp();
    WindowAdapter wa = new WindowAdapter() {
        @Override
        public void windowClosing( WindowEvent e ) {
            System.exit( 0 );
        }
    };
    addWindowListener( wa );
}


// needs to be protected if subclassed
private PlayerView getPlayerView( Player p ) {
    PlayerView v = new PlayerView( p );
    v.setBackground( FOREST_GREEN );
    return v;
}

   // needs to be protected if subclassed
private void setUp() {

    BlackjackDealer dealer = getDealer();
    PlayerView v1 = getPlayerView( dealer );
    this.Dealerview = v1;

    GUIPlayer human = getHuman();
    PlayerView v2 = getPlayerView( human );

    PlayerView [] views = { v1, v2 };
    addPlayers( views );


    dealer.addPlayer( human );

    addOptionView( human, dealer );
}

public PlayerView getDealerView(){
    return this.Dealerview;
}



// needs to be protected if subclassed
private void addPlayers( PlayerView [] p ) {        
    players.setBackground( FOREST_GREEN );
    for (PlayerView p1 : p) {
        players.add(p1);
    }  
    getContentPane().add( players, BorderLayout.CENTER );        
}

private void addOptionView( GUIPlayer human, BlackjackDealer dealer ) {

    OptionView ov = new OptionView( human, dealer );
    ov.setBackground( FOREST_GREEN );
    getContentPane().add( ov, BorderLayout.SOUTH );
}

private BlackjackDealer getDealer() {
    if( dealer == null ) {
        Hand dealer_hand = new Hand();
        Deckpile cards = getCards();
        dealer = new BlackjackDealer( "Dealer", dealer_hand, cards );

    }
    return dealer;
}

private GUIPlayer getHuman() {
    if( human == null ) {
        Hand human_hand = new Hand();
        Bank bank = new Bank( 1000 );
        human = new GUIPlayer( "Human", human_hand, bank );
    }

    return human;
}

private Deckpile getCards() {
    Deckpile cards = new Deckpile();
    for( int i = 0; i < 4; i ++ ) {
        cards.shuffle();
        Deck deck = new VDeck();
        deck.addToStack( cards );
        cards.shuffle();

    }
    return cards;
}

}