EDIT

function zen_dataportal_form_system_theme_settings_alter(&$form, &$form_state, $form_id = NULL)  {

  $which_image = "banner";

  $form['zen_dataportal'] = array(

      '#type' => 'fieldset',

      '#title' => t($which_image) . ' ' . t('image settings'),

      '#description' => t('If toggled on, the following image will be displayed.'),

      '#attributes' => array('class' => array('theme-settings-bottom')),

    );

    $form['zen_dataportal']['default_' . $which_image] = array(

      '#type' => 'checkbox',

      '#title' => t('Use the default') . t($which_image),

      '#default_value' => theme_get_setting('default_' . $which_image),

      '#tree' => FALSE,

      '#description' => t('Check here if you want the theme to use the image supplied with it.')

    );

    $form['zen_dataportal']['settings'] = array(

      '#type' => 'container',

      '#states' => array(

        // Hide the logo settings when using the default logo.

        'invisible' => array(

          'input[name="default_' . $which_image . '"]' => array('checked' => TRUE),

        ),

      ),

    );

    // If $path is a public:// URI, display the path relative to the files

    // directory; stream wrappers are not end-user friendly.

    $path = theme_get_setting($which_image . '_path');

    $image = '';

    if (file_uri_scheme($path) == 'public') {

      $url = file_create_url($path);

      $path = file_uri_target($path);

      $form['zen_dataportal']['settings'][$which_image . '_preview'] = array(

      	'#type' => 'item',

        '#title' => t('Preview'),

        '#markup' => '<div class="image-preview"><img src="' . $url . '"/></div>',

      );

    }

    $form['zen_dataportal']['settings'][$which_image . '_path'] = array(

      '#type' => 'textfield',

      '#title' => t('Path to custom') . ' ' . t($which_image),

      '#description' => t('The path to the file you would like to use as your') . ' ' . t($which_image),

      '#default_value' => $path,

    );

    $form['zen_dataportal']['settings'][$which_image . '_upload'] = array(

      '#type' => 'file',

      '#title' => t('Upload') . ' ' . t($which_image),

      '#maxlength' => 40,

      '#description' => t("If you don't have direct file access to the server, use this field to upload your image.")

    );

  $form['#validate'][] = 'zen_dataportal_theme_settings_validate';

  $form['#submit'][] = 'zen_dataportal_theme_settings_submit';

/**

 * Process zen_dataportal admin form submissions.

 */

function zen_dataportal_theme_settings_submit($form, &$form_state) {

  $values = &$form_state['values'];

  // If the user uploaded a new image, save it to a permanent location

  // and use it in place of the default theme-provided file.

  $which_image = "banner";

  if ($file = $values[$which_image . '_upload']) {

    unset($values[$which_image . '_upload']);

    $filename = file_unmanaged_copy($file->uri);

    $values['default_' .$which_image ] = 0;

    $values[$which_image . '_path'] = $filename;

    $values['toggle_' . $which_image] = 1;

  }

  if (!empty($values[$which_image . '_path'])) {

    $values[$which_image . '_path'] = _system_theme_settings_validate_path($values[$which_image . '_path']);

  }

  /*

   * Ok, we are done here the $values will be saved in the theme

   * variable by system_theme_settings_submit($form, &$form_state)

   */

}

/**

 * Validator for the system_theme_settings() form.

 */

function zen_dataportal_theme_settings_validate($form, &$form_state) {

  $validators = array('file_validate_is_image' => array());

  $which_image = "banner";

  // Check for a new uploaded logo.

  $file = file_save_upload($which_image . '_upload', $validators);

  if (isset($file)) {

    // File upload was attempted.

    if ($file) {

      // Put the temporary file in form_values so we can save it on submit.

      $form_state['values'][$which_image . '_upload'] = $file;

    }

    else {

      // File upload failed.

      form_set_error($which_image . '_upload', t('The image could not be uploaded.'));

    }

  }

}