var img = $("#img_id"); // Get my img elem

var pic_real_width, pic_real_height;

$("<img/>") // Make in memory copy of image to avoid css issues

    .attr("src", $(img).attr("src"))

    .load(function() {

        pic_real_width = this.width;   // Note: $(this).width() will not

        pic_real_height = this.height; // work for in memory images.

    });