the [post_thumb] shortcode just spits out the image source, you have to use that variable in context of standard HTML for showing an image, eg:
<img src=”[post_thumb size=’thumbnail’]” class=”alignleft” alt=”” />
I used single inverted commas indside the shortcode to specify the size, and double inverted commas outside, which worked, but I didn’t bother testing if that variation was necessary!
