Do you want the clients to get the web pages from your web server and then get the images directly from the file server? Or do you want to use the web server as kind of a "proxy" to the file server? For the latter, you'd either mount the shared disk on the web server and add the mount point to be accessible thru Apache config, or you could write a PHP/Perl/etc. script to pull the images from the file server (mounting it would be way easier).
For the former - if your file server is Windows or Samba and all your clients are Windows, you can put <img src="file://///ServerName\ShareName\file.jpg" ... and it will work because of the way Windows has automagic filesystem support for shared files. Useless if someone browses your web site with Linux. Also, Mozilla or Firefox need exactly the five slashes after file: and I think there is a about
:config option you need to set because it considers file links on a web site accessed via http a potential security hole, whereas IE will just let it work.
