Custom field type with imagefield
Found it: Forgot the 'input ...' in my custom theme function :).

function theme_mymodule_thumb_upload($variables) {
  $element = $variables['element'];

  if (isset($element['#file']-&gt;uri)) {
      $output = '&lt;div id="edit-logo-ajax-wrapper"&gt;&lt;div class="form-item form-type-managed-file form-item-logo"&gt;&lt;span class="file"&gt;';
      $output .= '&lt;img src="' . file_create_url($element['#file']-&gt;uri) . '" /&gt;';
      $output .= '&lt;/span&gt;&lt;input type="submit" id="edit-' . $element['#name'] . '-remove-button" name="' . $element['#name'] . '_remove_button" value="Remove" class="form-submit ajax-processed"&gt;';
      $output .= '&lt;input type="hidden" name="' . $element['#name'] . '[fid]" value="' . $element['#file']-&gt;fid . '"&gt;';

      return $output;
  }
}