mj-image-class.mjml

<mjml>
  <mj-body>
    <mj-section>
      <mj-column>
        <mj-image css-class="testing" src="http://www.online-image-editor.com//styles/2014/images/example_image.png" />
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>