imgTrueSize() : 画像の実サイズ取得関数
imgTrueSize = function(img){
 var w = img.width, h = img.height;
 if(typeof img.naturalWidth !== "undefined"){
  w = img.naturalWidth;
  h = img.naturalHeight;
 } else if(typeof img.runtimeStyle !== "undefined"){
  var run = img.runtimeStyle;
  var mem = { w:run.width, h:run.height };
  run.width = "auto";
  run.height = "auto";
  w = img.width;
  h = img.height;
  run.width = mem.w;
  run.height = mem.h;
 } else {
  var mem = { w:img.width, h:img.height };
  img.removeAttribute("width");
  img.removeAttribute("height");
   w = image.width;
   h = image.height;
  image.width = mem.w;
  image.height = mem.h;
 }
 return { width:w, height:h };
};