[hardwired]

image format that can have like transparent/ non-visible pixels on it
png works okay for this.
There may be some options depending on what you are trying to do. If loading two images and drawing them one-over-the-other/overlayed doesn't give you what you want you can draw them both into a BufferedImage to use in your app. Here's an example of how you can do it. I made up two images since you didn't sppecify any to work with.

Code:

import java.awt.*;
import java.awt.geom.*;
import java.awt.image.BufferedImage;
import javax.swing.*;

public class OverlayImages {
    private ImageIcon getContent() {
        return new ImageIcon(getImage());
    }

    private BufferedImage getImage() {
        BufferedImage bg = getOpaqueImage();
        BufferedImage fg = getTranslucentImage();
        int w = bg.getWidth();
        int h = bg.getHeight();
        int type = BufferedImage.TYPE_INT_RGB;
        BufferedImage image = new BufferedImage(w, h, type);
        Graphics2D g2 = image.createGraphics();
        g2.drawImage(bg, 0, 0, null);
        int x = (w - fg.getWidth())/2;
        int y = (h - fg.getHeight())/2;
        g2.drawImage(fg, x, y, null);
        g2.dispose();
        return image;
    }

    private BufferedImage getOpaqueImage() {
        int w = 200;
        int h = 140;
        int type = BufferedImage.TYPE_INT_RGB;
        BufferedImage image = new BufferedImage(w, h, type);
        Graphics2D g2 = image.createGraphics();
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setBackground(Color.orange);
        g2.clearRect(0,0,w,h);
        g2.setPaint(Color.green.darker());
        g2.draw(new Line2D.Double(w/16.0,h/16.0,w*15/16.0,h*15/16.0-1));
        g2.draw(new Line2D.Double(w*15/16.0,h/16.0,w/16.0,h*15/16.0-1));
        g2.setPaint(Color.blue);
        g2.draw(new Rectangle2D.Double(w/16, h/16, w*7/8, h*7/8));
        g2.dispose();
        return image;
    }

    private BufferedImage getTranslucentImage() {
        int w = 90;
        int h = 90;
        int type = BufferedImage.TYPE_INT_ARGB_PRE;
        BufferedImage image = new BufferedImage(w, h, type);
        Graphics2D g2 = image.createGraphics();
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        Ellipse2D e = new Ellipse2D.Double(10,10,w-20,h-20);
        g2.setPaint(Color.red);
        g2.fill(e);
        g2.setPaint(Color.blue);
        g2.draw(e);
        g2.dispose();
        return image;
    }

    public static void main(String[] args) {
        OverlayImages test = new OverlayImages();
        ImageIcon icon = test.getContent();
        JOptionPane.showMessageDialog(null, icon, "",
                           JOptionPane.PLAIN_MESSAGE);
    }
}