select exact size of uploaded images

Hi, i’m having some problems in getting the url of custom sizes images.
I have a custom sized image type, named dimension-gallery (hard cropped, 800×533).
I’m trying to get the url formatted in this way:
wp-content/uploads/2013/09/photo-800×533.jpg
But no luck.
I’m using:

<?php
$imgs  = get_images_src('dimension-gallery', true);
foreach( $imgs as $i )
echo '<li style="background-image: url('. $i[0].');">></li>';
?>

but it points to the uncropped-original url (wp-content/uploads/2013/09/photo.jpg)

if i echo $i[1].$i[2] the result is 800533 , correct;

anyway i can’t get the desired url

Thanks

http://wordpress.org/plugins/multi-image-metabox/