<?xml version="1.0" encoding="UTF-8"?>
<ProductCatalog>
  <Product>//тут есть Image
    <Image>img_1</Image>
    <Vendor>Tra-Ta-ta</Vendor>
    <Size>Tra-Ta-ta</Size>
 </Product>
   <Product>//а тут нету Image
    <Vendor>Tra-Ta-ta</Vendor>
    <Size>Tra-Ta-ta</Size>
 </Product>
</ProductCatalog>

$xml = simplexml_load_file("file.xml") or die("url not loading");
foreach($xml->Product as $child)
  {
  echo $child->Image; 
  if($child->Image) 
  	{
		echo"Есть картинка <br>";
	} else
	{
		echo "Нет картинки <br>";
	};
  
  }