<?php
// Put in this array every substring that identifies the URL of an image search:
$hr = array("images.google", "images.search.yahoo.com", "search.live.com/images/");

// Assign the requested URL to $imgpath (you may want to add some extra input security check here)
$imgpath = $_GET["imgpath"];

// Loop the array
foreach($hr as $imgreferrer) {

  $found = strpos($_SERVER["HTTP_REFERER"], $imgreferrer); // Check the actual referer against the given ones

    if ($found !== false) {       // if an Image Search URL is detected in the HTTP REFERER value, show your landing page:
      header("Content-type: text/html");
      ?>

      <!-- put here the HTML code of your landing page (or place here a redirect to the landing itself): -->
      <h1>Image embed code: [HTML with image and backlink code follows]</h1>
      <img src="<?php echo $imgpath; ?>" alt="" />

      <?php
      die(); // Don't go further in the Loop
    }
}
// Else, continue to serve the image regularly:

header("Content-type: image/jpeg"); // MAKE SURE that the content type matches your image's one here
echo file_get_contents($imgpath); // MAKE SURE you can open URLs in PHP with file_get_contents()

?>