$arr = wp_get_attachment_image_src(
$str = str_replace("[post_thumb]",
Why "single-post-thumbnail" as second parameter of wp_get_attachment_image_src?
If your theme does not have that "single-post-thumbnail" size defined , all you get is a huge image :-)
I would change it in "thumbnail" or even better add parameter support to define the size in [post-thumb] shortcode.