Prefix html before radiobutton (input field)
You can override the theme_radio function in your own theme :

function mytheme_radio($variables) {
  $element = $variables['element'];
  $element['#attributes']['type'] = 'radio';
  element_set_attributes($element, array('id', 'name','#return_value' =&gt; 'value'));

  if (isset($element['#return_value']) &amp;&amp; $element['#value'] !== FALSE &amp;&amp; $element['#value'] == $element['#return_value']) {
    $element['#attributes']['checked'] = 'checked';
  }
  _form_set_class($element, array('form-radio'));

  if ($its_my_field) {
     return '&lt;span class="radio" style="background-position: 0px -28px;"&gt;&lt;/span&gt;&lt;input' . drupal_attributes($element['#attributes']) . ' /&gt;';
  }
  else {
     return '&lt;/span&gt;&lt;input' . drupal_attributes($element['#attributes']) . ' /&gt;';
  }


Maybe try with the more generic function theme_form_element.

Make sure to add conditions to only affect the form or the element you need like in the example written above.