If you develop content management sites or themes, such as for WordPress, here are a couple of  tricks that will make your visitor’s lives a little bit easier.  When people post a link to one of your blog’s posts, Facebook tries to determine the best image thumbnail, title, and description for the link being posted.  Luckily, there are ways you can influence this automated process to handle your blog cleanly, and reflect the content and imagery that you want displayed.  We’ll cover the title and description first, then move on to some advanced thumbnail management.

Title and Description

First up, the All in One SEO Pack is a WordPress plugin that optimizes your blog for search engines.  Using this plugin provides meta tags for searching, a side effect of which is that Facebook will also use them to populate the user’s post.  Installing this plugin is the easiest way to use meta tags, as they are automatically created based on your posts title and description.  A few of the settings are shown below.

Thumbnails

The second item, thumbnail selection, can be a bit more complicated, and may require some knowledge of PHP. While there are WordPress tools that enable manual thumbnail selection, to have full automatic selection you need customized code in your theme. When Facebook looks for eligible thumbnails, it looks for images with certain file size and resolution constraints.  This eliminates background images or small icons from the selection.  It then orders the images from largest to smallest by file size.  This algorithm is effective, but it doesn’t always place your key image first in line.

Take nowsci.com for example.  Above this article, you’ll see an image pulled from Facebook’s homepage.  We include an image or video with every article we publish, but these images are too large for Facebook to grab.  Usually, someone posting a link to this page on Facebook would get one of the images from our menus first on the list, followed by the rest of the menu images and a spacer.

There are two ways to handle thumbnails, depending on your needs.  Ask Insiva has identified one easy methodology. If you only want a single image to be displayed when a person posts your site on Facebook, and that image fits within the constraints that Facebook searches, you can use the image_src meta tag:

      <link rel="image_src" href="http://domain.ext/image.png" />

This works, but has limitations.  If your post has more than one image, it will not allow the user to select the image they wish to use.  Also, if your images are larger than what Facebook allows, no thumbnail will show up at all.

To solve this in a theme that needs this functionality automatic, we have to use PHP. First, lets cover off on images that are too large for Facebook to grab.  You can use a PHP script like the one below to resize your images as thumbnails.  In this example, we will crop out a center section of point x=100,y=100 to x=400,y=400 from our source images, and resize them into 200x200px thumbnails.  This example will work in both PHP4 and PHP5.

      <?php
        $imgfile = $_GET['img'];
        $cropW   = 200;
        $cropH   = 200;

        // Create two images
        $origimg = imagecreatefromjpeg($imgfile);
        $cropimg = imagecreatetruecolor($cropW,$cropH);

        // Get the original size
        list($width, $height) = getimagesize($imgfile);

        // Crop and display
        imagecopyresized($cropimg, $origimg, 0, 0, 100, 100, $cropW, $cropH, 400, 400);
        header("Content-type: image/jpeg");
        imagejpeg($cropimg);

        // destroy the images
        imagedestroy($cropimg);
        imagedestroy($origimg);
      ?>

Next, we’ll use this script with the above meta tag to display the thumbnail on Facebook:

     <link rel="image_src" href="thumb.php?img=http://domain.ext/image.png" />

But what if we want to display more than one image in the pick list on Facebook? To do this, we must determine the visiting client’s user agent.  A user agent defines what browser someone is visiting your site with.  You can view your user agent by visiting whatsmyuseragent.com. Luckily for us, when Facebook tries to grab a page, they display a unique user agent containing the string “facebookexternalhit/1.0″. By searching through the PHP variable $_SERVER['HTTP_USER_AGENT'] for this string, you can identify if Facebook is querying the page.  Once identified, you can modify your theme’s output based on the user agent string.  For instance, you could include only the key image for your post without displaying anything else if “facebookexternalhit/1.0″ exists.

For the sake of space we’ll walk you through a very basic implementation of this, instead of going into all the details of theme integration.  Take a look at the below source code.

      <html>
        <head>
          <title>Test</title>
        </head>
        <body>
          <img src="bad-image.jpg"> <!-- This image is 10K -->
          <img src="small-header-image.jpg"> <!-- This image is 20K -->
          <!-- Below image is too large for Facebook -->
          <img src="large-header-image.jpg">
          This is the body copy for my post.
        </body>
      </html>

In the above example, when Facebook grabs this page, it would select small-header-image.jpg as the default headline image, followed by bad-image.jpg in the pick list.  To force Facebook to display large-header-image.jpg first, and then only small-header-image.jpg next, you would use the following code:

      <?
        $facebook = 0;
        $userAgent = $_SERVER['HTTP_USER_AGENT'];
        if (strpos($userAgent,"facebookexternalhit") == 0) {
          $facebook = 1;
        }
      ?>
      <html>
        <head>
          <title>Test</title>
        </head>
        <body>
          <? if ($facebook == 0) { ?>
            <img src="bad-image.jpg"> <!-- This image is 10K -->
            <img src="small-header-image.jpg"> <!-- This image is 20K -->
            <!-- Below image is too large for Facebook -->
            <img src="large-header-image.jpg">
            This is the body copy for my post.
          <? } else { ?>
            <img src="small-header-image.jpg"> <!-- This image is 20K -->
            <img src="thumb.php?img=large-header-image.jpg">
          <? } ?>
        </body>
      </html>

This would display large-header-image.jpg as a thumbnail to Facebook, followed by small-header-image.jpg when it grabs the page. The one drawback to this methodology is using caching plug-ins such as WP Super Cache.  Many caching systems do not use user agents to determine if a new page needs to be served up, because the standard HTML will work for almost all browsers.  However, when advanced caching like Super Cache is enabled, it can cause Facebook to grab the cached HTML, rather than go back through the PHP code to generate the page.

If you would like more details on how to accomplish these tweaks within your site, feel free to post in the comments.