org.zkoss.zul.Image to byte array?

I'm struggling trying to save an image in postgresql database using a byte array.

The problem is, i'm trying to convert the image to byte array and when I save it gives me this error

ERROR: column "avatar" is of type bytea but expression is of type bigint

this is the code I'm using to convert the image

 public static byte[] getBytes(Object obj){
    byte[] data = null;
    try {
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        BufferedImage bufferedImage = new BufferedImage(image.getWidth(null), image.getHeight(null), 1);
        ObjectOutputStream oos = new ObjectOutputStream(bos);
        oos.writeObject(obj);
        oos.flush();
        oos.close();
        bos.close();
        data = bos.toByteArray();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    return data;
}

Any help will be appreciated :)

PS: Sorry for the bad english