I needed to change image style on the user page, and I also needed to theme some fields. I was able to do everything I needed in the .theme file and the user twig template file.I created a new style ‘user_image’ then in the YOURTHEME.theme file I created a new image url for the image in the hook_preprocess_user.

function YOURTHEME_preprocess_user(&$vars) {

  $user = $vars['user'];
  // get user image
  if (!empty($user->user_picture[0])) {
    $cover_image = $user->user_picture[0]->entity->getFileUri();
    $image_url = ImageStyle::load('user_image')->buildUrl($cover_image);
    $vars['user_pic'] = $image_url;
    // get user name
    $vars['user_name'] = $user->getUsername();
  }

Then in s the user.html.twig I had the two extra variables I could play with.

<div class="row">
  <div class="col-sm-8">
    {% if content %}
      <div class="card">

        <div class="row">
          <div class="col-sm-3 pull-left">
            {% if user_pic %}
              <img style="min-width: 100%;" class="img-circle img-thumbnail" src="{{ user_pic }}">
              {# {{ content.user_picture }} #}
            {% endif %}
          </div>

          <div class="col-sm-9">
            <h1>{{user_name}}</h1>

              {# other fields here  #}

          </div>
        </div>

      </div>
    {% endif %}
  </div>
  <div class="col-sm-4">
  </div>
</div>

Cheers!