Everyone already wanted to put an image on a Combo Box, whether to show country flags, avatars or even simple icons. I'll show you how to do this in Java Swing using JComboBox.

We need to create two classes:

import javax.swing.ImageIcon;
import javax.swing.JComboBox;

@SuppressWarnings("rawtypes")
public class LanguagesComboBox extends JComboBox {

	private static final long serialVersionUID = 1L;

	private ImageIcon img;

	private String[][] LIST_LANGUAGES = new String[][] { { "pt", "pt.jpg" }, { "en", "en.jpg" } };

	@SuppressWarnings("unchecked")
	public LanguagesComboBox() {
		for (String[] item : LIST_LANGUAGES) {
			img = new ImageIcon(getClass().getResource("/images/flags_language/" + item[1]));
			img.setDescription(item[0]);
			this.addItem(img);
		}
		LanguageComboBoxRenderer renderer = new LanguageComboBoxRenderer();
		setRenderer(renderer);
		setEditable(false);
	}

	public void setSelectedLanguage(String value) {
		for (int i = 0; i < LIST_LANGUAGES.length; i++) {
			if (value.equals(LIST_LANGUAGES[i][0])) {
				img = new ImageIcon(getClass().getResource("/images/flags_language/" + LIST_LANGUAGES[i][1]));
				img.setDescription(LIST_LANGUAGES[i][0]);
				setSelectedIndex(i);
			}
		}
	}
}

import java.awt.Component;
import java.awt.Image;

import javax.swing.ImageIcon;
import javax.swing.JLabel;
import javax.swing.JList;
import javax.swing.ListCellRenderer;

@SuppressWarnings("rawtypes")
class LanguageComboBoxRenderer extends JLabel implements ListCellRenderer {
   private static final long serialVersionUID = 1L;
	public LanguageComboBoxRenderer() {
        setOpaque(true);
        setHorizontalAlignment(LEADING);
        setVerticalAlignment(CENTER);
    }
    public Component getListCellRendererComponent(
        JList list,
        Object value,
        int index,
        boolean isSelected,
        boolean cellHasFocus)
    {
        if (isSelected) {
            setBackground(list.getSelectionBackground());
            setForeground(list.getSelectionForeground());
        } else {
            setBackground(list.getBackground());
            setForeground(list.getForeground());
        }
        ImageIcon icon = (ImageIcon)value;
        Image img = icon.getImage().getScaledInstance(30, 22, Image.SCALE_DEFAULT);
        setText(icon.getDescription());
        setIcon(new ImageIcon(img));
        return this;
    }
}

Example of Use:

LanguagesComboBox cbLanguages = new LanguagesComboBox();
YOUR_PAINEL.add(cbLanguages);

The images used in this example need to be in this path: src/images/flags_language/

Click Here to download the images used in this example.