If your users are using FileMaker Pro to connect to the database (not a web browser), you can store the images "by reference" on the server as long as you share the directory where the files are located and map/mount that directory on each client machine in exactly the same way. The file path used by the user to access the image files directly must be exactly the same filepath stored in the container.
As long as it's not an "NFS server", the client's only need "read" access permissions on this shared folder and they should be able to see the images in the container field. A bug currently requires "write" access on NFS servers.
A web viewer can be used to display the images also. They need the same access just described for using the container so I don't see the advantage here to using that option. If you do use a webviewer, I think your URL would need to start with file: instead of imagewin:--a tag unique to use by container fields--not web browsers and viewers.
The images are stored by reference. That is the problem. Only by creating something like a VPN can a drive be mounted for someone in Los Angeles viewing the files in Sacramento. Unfortunately tests at another location show this to be way too slow to display images and worse in the foothills of california where we are, ATT requires their router be used that has no VPN access/help.
I had hoped that using a web viewer could bypass the need to mount a drive in order to view images stored by reference remotely. The "imagewin" was just an example of URL attempts. I tried every possible way I could find to resolve a path to no avail. I had hoped to find someone who might have had similar constraints that could view referenced images remotely.
Hmm, a lot of info there that would have changed my suggestions had they been in your first post...
The following comments are not based on personal experience--but rather on what I know is possible and in part on posts made by others here in the forum:
Only a few options left if you can't use VPN. (I'd look at using downsampled, lower resolution images if possible and maybe a local FileMaker inteface file connecting to the hosted data file to get faster results.)
Publish to the web so that they can use a web browser to access the images (I wonder if the image files could be published and the database then used a webviewer to the URL of the image file's web page...)
Don't use Store a Reference
You might also consider using a Hosting service as they won't have the limitations you have at your site. Don't know what they'd say to allowying VPN connections, but it might be worth asking them about that option.