get URLs of images from gallery? how to?

I wish to use a form of preloader to work with our our CDN services.

To do so , I need to build a list of the image URLS INSIDE a gallery.

How do I get these?

the plan is simple.
set up a css class as hidden,
build list of image URL’s to load, compose this as html with hidden class,

I can see no facility for getting the attachment URLS from all images in a gallery, I can only figure out how to get the post_thumbnail.

cheers.
Vince.