One of the improvements in WordPress 2.9 is native support for thumbnail images on posts. The API is pretty good – although it assumes that you want to return the whole IMG tag. In my case I wanted to return the URL of the image (So I could wrap the actual thumbnail in a lightbox-style link to the larger versions).

So – if you want to get the URL of your posts thumbnail – here’s the code you need:

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$image_id = get_post_thumbnail_id();
$image_url = wp_get_attachment_image_src($image_id,'large');
$image_url = $image_url[0];
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