Remove width and height attributes from post image

I don't want that post image is appointed by width and height. Output at the moment:

<img src="image.jpg" alt="image" width="150" height="150" />

It must be:

<img src="image.jpg" alt="image" />

or

<img src="image.jpg" alt="image" width="100%" height="auto" />

I've tried to use the following jQuery script but it doesn't help:

$(document).ready(function(){
  $('.single-page-content').find('img').removeAttr('width');
});

In addition, I'm using the following CSS code:

.content .single-page img {
  max-width: 100% !important;
  min-width: 100% !important;
  height: auto !important;
}