I have:

img {
  max-width: 100%;
  height: auto;
  border: 0;
}

insert image does overides height: auto of my style.css and ery large images are distorted.
I did wrote img in Additional CSS classes but nothing.
What should I do to have responsive images when I choose to insert Default insert style:original image?

Comments

Insert doesn't add any inline styles to its inserted images and it doesn't include any CSS that affects content. My guess is this is a problem with your WYSIWYG setup regardless of your use of Insert module.