WordPress Images | How to get the cropped image?

if you have the attachment (image) id, you ca use :

 wp_get_attachment_image_src( $attachment_id, $size);

where $size is the smaller version you want. (see http://codex.wordpress.org/Function_Reference/wp_get_attachment_image_src)

If you don’t know the attachment id you may query the database for full image size url :
http://pippinsplugins.com/retrieve-attachment-id-from-image-url/