You can change the label using the template_preprocess_field hook.
For instance, if your field was named "phone" and the content type was "business", you could add this to your template.php:
template_preprocess_field (&$variables) {
$content_type = $variables['element']['#bundle'];
$field_name = $variables['element']['#field_name'];
if ($content_type == 'business' && $field_name == 'phone') {
$variables['label'] = '<img src="http://www.carloancentre.ca/wp-content/uploads/2014/03/Contact-Methods-Phone-icon.png" height="16" width="16"/>' . $variables['label'];
}
}