Add a class to images in drupal
Few ways to do this:


JavaScript. Not the best one, but fastest:

$('img').addClass('adaptive-image');

For styling field images, see the response by Manoj Bisht.
For styling images added through WYSIWYG you will have to write a custom text filter and add classes. Depending on the WYSIWYG editor you use, you might be able to create a plugin / set configuration so that all images inserted automatically get certain CSS class.
Ultimately, you can just adjust the CSS of .adaptive-image, or clone the styles and apply them for all img tags.
Example source:

.adaptive-image {
  /* Whatever. */
}


Change to, or add this:

img {
  /* Copy styles from .adaptive-image. */
}