In response to his question here, assuming your content is in a string called $content:

$num_paras = substr_count($content, '<p>'); 
switch ($num_paras) {
  case 1:
    $image = 'one.jpg';
    break;
  case 2:
    $image = 'two.jpg';
    break;
  case 3:
    $image = 'three.jpg';
    break;
  default:
    $image = '';
    break;
}