Looping custom sized post images

So i’ve read the codex here
http://codex.ww.wp.xz.cn/Function_Reference/add_image_size

and I specified image sizes in my functions.php and it looks good.

What i’m trying to do now is this:
in a loop, go through posts of a certain category and list out the thumbnails of these specified sizes.

For what I need, I need to pull the URL’s of the specific new image sizes.

can anyone point me in the right direction or clue me into what function i need to use to call my image URLs?

<?php if ( has_post_thumbnail() ) { the_post_thumbnail( ‘category-thumb’ ); } ?>

i’ve seen this, but that only posts the image. I need to do that except it posts the URL of the image.

hope that makes sense.