You can detect the path with one of the "get container attributes" functions and based on that path and the OS of the user, calculate the proper path and get the file that way.
A simpler solution would be to use FMS and let that do the container data management.
Thanks for the suggestion but FMS is just to expensive for the client.
I'm testing the following solution:
A textfield (global) that stores the "base path" that's equal for Mac en Windows.
A second textfield that holds the name of the pict.
A unstored calculation field, result = container, where I store a reference to the pict, calculated for Mac or Windows.
Works cross platform but you're right using FMS would be easier.