var selectedImage;
    
function on(image, show) {
    if (image == null) 
        image = selectedImage;
    var l = image.src.length;
    var i = image.src.indexOf(".",l-4);            
    if (show) {
        selectedImage = image;
        image.src = image.src.substring(0,i) + "_a." + image.src.substring(i+1,l);
    } else {
        image.src = image.src.substring(0,i-2) + "." + image.src.substring(i+1,l);
    }     
}