function swapImage(image_id) {
    thumb_img = document.getElementById(image_id);
    main_img = document.getElementById('main_img');
    
    thumb_file = thumb_img.src.match(/[\d]+\.[a-z]{2,4}$/i);
    main_file = main_img.src.match(/[\d]+\.[a-z]{2,4}$/i);
        
    thumb_img.src = thumb_img.src.replace(/[\d]+\.[a-z]{2,4}$/i, main_file);
    main_img.src = main_img.src.replace(/[\d]+\.[a-z]{2,4}$/i, thumb_file);
}