var current = 'f0';
var fading = false;
var idOfElement;

function showLinkImage(id) {
    if (id != current && !fading) {
        fading = true;
        new Effect.Parallel([
            new Effect.Fade(current),
            new Effect.Appear(id)],
        {
            duration: 0.5, afterFinish: function(currentSetzen) { current = id; fading = false; }
        });
    }
}