WordPress 2.9 Changes the Post Image Function

A change that may break your theme

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In the latest WordPress version, there has been a change in how post images are handled. As of V2.9, the_post_image() has been replaced with the_post_thumbnail(). That’s right, replaced, not in addition to. The new function has some interesting features as well.

Defining Thumbnail Sizes

Set both max sizes in the function like this:
the_post_thumbnail(array(100,150));
This defines the thumbnail as 100px wide max or 150px high max. Maintains aspect ratio.

Set a style class

Set a style class in the function like this:
the_post_thumbnail(array(100,150), array('class' => 'alignleft'));
or add your own class like this:
the_post_thumbnail(array(100,150), array('class' => 'alignleft custom_class'));

Typical usage

In the loop, add the post thumbnail along with the excerpt:
while (have_posts()) : the_post();
the_post_thumbnail(array(100,100), array('class' => 'alignleft'));
the_excerpt();
endwhile;

Enable it in your theme function file

Another quirk that’s significant is this function isn’t enabled by default. It has to be enabled in your theme function file. To make these new functions work, add this to functions.php:
add_theme_support( 'post-thumbnails' );
Without this line the post thumbnails functions won’t work.

Add a Post Thumbnail

With V2.9 there is now a place to add a post thumbnail. The thumbnail is not drawn from the content of the post, it stands alone. If a thumbnail isn’t chosen, the_post_thumbnail() function won’t render a thumbnail.

Conclusion

Much more control over thumbnails in V2.9, but there’s a chance this new function may break your existing theme or a plugin. More control over what is used as a thumbnail, but you have to define the thumbnail. Test carefully

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