function RollOverImage(img){
    this.img = img;
    this.originalPath = this.img.src;
}

//マウスオーバー・マウスアウト時処理
RollOverImage.prototype.setMouseOverImage = function(filepath){
    this.mouseoverImg = new Image();
    this.mouseoverImg.src = filepath;
    var originalPath = this.originalPath;

    this.img.onmouseover = RollOverImage.createHandler(this.img, filepath);
    this.img.onmouseout = RollOverImage.createHandler(this.img, this.originalPath);
}

//マウスダウン・マウスアップ時処理
RollOverImage.prototype.setMouseDownImage = function( filepath ){
    this.mousedownImg = new Image();
    this.mousedownImg.src = filepath;
    
    this.img.onmousedown = RollOverImage.createHandler(this.img, filepath);
    this.img.onmouseup = RollOverImage.createHandler(this.img, this.originalPath);
}

//画像入れ替えを実行するクロージャ作成
RollOverImage.createHandler = function(img, path){
    return function(){
        img.src = path;
    }
}