I also made a posting about it here: http://suhanto.net/the-importance-of-featured-image-in-wordpress/. It is not as thorough as this article, but it takes different focus.

Thanks for this detailed tutorial, and for Mark's tutorial too. But this isn't working for me, and I've spent a week researching and trying everything I can to get it to work.

I am trying to build a gallery. The idea is that the first three images from each post in a specific category (Gallery) are displayed as a hard cropped thumbnail. I can get the images to be cropped, but not hard cropped, the aspect ratio is maintained. I've followed very carefully the advice here and on Mark's blog. It seems that I am not the only one having this issue, but no one seems to have a solution.

I am using WordPress 2.9.2 on a Mac and In my functions.php file I have:

add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 176, 176, true );
add_image_size ('gallery', 150, 150, true);

Then in my template (page.php) I have:

<?php echo wp_get_attachment_image($attachment_id, $size = 'gallery', $icon = false );

The images are displayed with the largest dimension set at 150, and then the other dimension is less than 150. If I've made an obvious mistake, or anyone has an idea why the hard crop isn't working, I'd really like to know. Thanks In Advance.

Most folks want to be able to determine what those thumbnails will display as, and if you give them the option to fill it out as 150x120 and it shows as 120x120 - that's a support nightmare.

Good news — you can force a "hard crop" to 150x120. The example used in this post isn't ideal, because WordPress can't (yet) do on-the-fly resizing/cropping. What it's doing if you use the above example is browser-resizing. And if you're doing that then no, you can't specify a hard crop.

But if you define your sizes "ahead of time" in your theme's functions.php, WP will generate these sizes (including a hard-crop version if you ask for it), and then in the theme, it will use the size you ask for, without browser resizing.

I've written up detailed info on how to implement this feature properly.

Important for this function, it is only working when it is explicitly enabled in the theme.

Does this mean that unless the theme specifies this, you can't use the thumbnail feature? Like if a plugin wants to display the post thumbnail, it can't?