function toggleImage( a ) {
	var images = [ '1.png', '2.png', '3.png' ];
	a.src = images[ images.indexOf( a.src ) + 1 ];
}

toggleImage( this );

function toggleImage( a ) {
	var images = [ '1.png', '2.png', '3.png' ];
	a.src = images[ images.indexOf( a.src ) == images.length ? 0 : images.indexOf( a.src ) + 1 ];
}

var images = ["1.png", "2.png", "3.png"];

function toggleImage(a) {
    images.push(images.shift());
    a.src = images[0]
};

function nextImage( a ){
    if( ! nextImage.images ){
        nextImage.images = [...];
        nextImage.i = 0;
    }
    nextImage.i = (nextImage.i+1) % nextImage.images.length;
    a.src = nextImage.images[nextImage.i];
}

function nextImage(a) {
    if (!nextImage.images) {
        nextImage.images = ["1.png", "2.png", "3.png", "4.png"];
        if (nextImage.images.length > 2) nextImage.images = nextImage.images.concat(nextImage.images.slice(1, nextImage.images.length - 1).reverse());
        nextImage.i = 0
    }
    nextImage.i = (nextImage.i + 1) % nextImage.images.length;
    a.src = nextImage.images[nextImage.i]
};

var Images = new Class({
    Implements: Options,

    initialize: function( options ){
        this.setOptions(options);
        var self = this;
        this.options['prev-link'].addEvent('click', function(){
            return self.prev.call(self);
        });
        this.options['next-link'].addEvent('click', function(){
            return self.next.call(self);
        });
    },

    next: function(){
        this._i = (this._i + 1) % this.options['images'].length; 
        this.options['img'].src = this.options['images'][this._i];
    },

    prev: function(){
        this._i--;
        if( this._i < 0 )
            this._i = this.options['images'].length-1;
        this.options['img'].src = this.options['images'][this._i];
    }
});

initialize: function( options ){
    ...
     this.options['prev-link'].addEvent('click', this.prev.of(this));
     this.options['next-link'].addEvent('click', this.next.of(this));
},