code: def preview_box(endpoint=None, filename=None):
          
          preview_size = current_app.config['AVATARS_CROP_PREVIEW_SIZE'] or current_app.config['AVATARS_SIZE_TUPLE'][2]
  
          if endpoint is None or filename is None:
              url = url_for('avatars.static', filename='default/default_l.jpg')
          else:
              url = url_for(endpoint, filename=filename)
          return Markup('''
          <div id="preview-box">
          <div class="preview-box" style="width: %dpx; height: %dpx; overflow: hidden;">
            <img src="%s" class="jcrop-preview" alt="Preview"/>
          </div>
        </div>''' % (preview_size, preview_size, url))
docs: Create a preview box.
  
          :param endpoint: The endpoint of view function that serve avatar image file.
          :param filename: The filename of the image that need to be crop.