Absolute center an image [JS works sometimes]

(function(){

    var partenaires = {

        init: function() {
            this.centerImg();
        },

        centerImg: function() {

            $('.partenaires').find('img').each(function() {
                console.log(this);
                //On récup la dimension de l'image
                var $this = $(this),
                widthImg = $this.width(),
                heightImg = $this.height();

                //On centre l'image
                $this.css({
                    'top':'80px',
                    'left': '160px',
                    'margin-left':-widthImg/2,
                    'margin-top': -heightImg/2
                });

            });
        }// centerImg()
    };

    partenaires.init();

})();