Together with two other developers I have spent a good deal of the last two years building a system with a similar purpose and functionality and here are some of my thoughts
First of all, get FileMaker Server Advanced. Sharing through FM Pro is a nice feature for the smallest workgroup, but it really is limited for doing what you are after.
Secondly, in order for IWP users to access the images through the filesystem you do need them to mount a share, similar, or identical to how your inhouse users are doing it. You do not say if IWP users are inside or outside of your firewall. You will probably have trouble convincing the IT department that it is a good idea to open up filesharing to external users. If that is not an option you will have to serve the images in some other way, either by embedding them in a database, or, by using some other means. In our case we are using PHP and web viewer controls.
Third, with FM Server Advanced you can tie your FM databases to an Active directory / Open directory server through LDAP, which will bring true authentication. You should probably also study FileMakers security model, and custom access rights etc.
If all you need is to share the database with a few inhouse co-workers, then keep it simple and adjust your expectations to what can reasonably be achieved