Validate the submitted GD settings.
function image_gd_settings_validate($form, &$form_state) {
// Validate image quality range.
$value = $form_state['values']['image_jpeg_quality'];
if (!is_numeric($value) || $value < 0 || $value > 100) {
form_set_error('image_jpeg_quality', t('JPEG quality must be a number between 0 and 100.'));
}
}