Is it possible to point /images folder out of 'multisites' for each child site?

We'd like to point images to the master site's images folder (with sub folder created for each child site).

Concerning the /images, it is NOT recommended to change its path.
The reason is that now, most of the extension may save their own images inside the /images directory.
Therefore, it is recommended to keep this directory shared.

What you need to do is to setup another /images_specific directory that will NOT be shared between the sites and would become specific for each site.

See the Media Manager / Option that allow modifying the default /images directory and also the JMS template (or manage) rule / folder accordeon to change the image path.
As this parameter is saved in the DB, it is only available when copying (or sharing) a website. It has no effect when using "Fresh Slave".