How do I alter images being added via IMCE CKEditor plugin?
So, here is what I did, even if maybe its dirty.

Add a new module which attaches a custom library to forms:

function mymodule_form_alter(&amp;$form, $form_state) {
  $form['#attached']['library'][] = 'mymodule/form';
}


In the mymodule.libraries.yml:

form:
  version: 1.x
  js:
    js/mymodule.js: {}
  dependencies:
    - core/ckeditor
    - ckeditor/drupal.ckeditor
    - imce/drupal.imce
    - imce/drupal.imce.ckeditor


And in the mymodule.js I pretty much just re-write the imce sendto function:

if (undefined !== CKEDITOR &amp;&amp; CKEDITOR.imce.sendto) {
  CKEDITOR.imce.sendto = function (File, win) {
    var imce = win.imce;
    var editor = CKEDITOR.instances[imce.getQuery('ck_id')];
    if (editor) {
      var i;
      var text;
      var lines = [];
      var selection = imce.getSelection();
      var is_img = imce.getQuery('type') === 'image';
      for (i in selection) {
        if (!imce.owns(selection, i)) {
          continue;
        }
        File = selection[i];
        // Image
        if (is_img &amp;&amp; File.width) {
          lines.push('&lt;img class="img-responsive" src="' + File.getUrl() + '" /&gt;');
        }
        // Link
        else {
          // Use the selected text/image for the first link
          text = !lines.length &amp;&amp; CKEDITOR.imce.getSelectedHtml(editor) || File.formatName();
          lines.push('&lt;a href="' + File.getUrl() + '"&gt;' + text + '&lt;/a&gt;');
        }
      }
      editor.insertHtml(lines.join('&lt;br /&gt;'));
    }
    win.close();
  };
}


Hope this is helpful to somebody. And maybe there is another, better way of doing this - glad for any input!