function imgFix() {
  //定义要限制的图片宽高,这个宽高要同style里面定义的相同，小于限定高宽的图片不操作
  var widthRestriction = 600;
  var heightRestriction = 450;
  var allElements = document.getElementsByTagName('*')  
  for (var i = 0; i < allElements.length; i++) {
    if (allElements[i].className.indexOf('imgBox') >= 0) {
      var imgElements = allElements[i].getElementsByTagName('img');
      for (var j=0; j < imgElements.length; j++) {
        if ( imgElements[j].width > widthRestriction || imgElements[j].height > heightRestriction ) {
          if ( imgElements[j].width > imgElements[j].height) {
            imgElements[j].height = imgElements[j].height*(widthRestriction/imgElements[j].width);
            imgElements[j].width = widthRestriction;
          } else {
               imgElements[j].width = imgElements[j].width*(heightRestriction/imgElements[j].height);
            imgElements[j].height = heightRestriction;
          }
        }
      }
    }
  }
}
window.onload = imgFix;