When you change the storage to external, FM try to transfer contents, but file reference is not transferred.
If you mean "picture path locations were provided with a script." as embedded pictures, they will be transferred, but setting field with image: path string don't embed.
My database was ported over to a Mac running FileMaker Pro from a PC running Access 2010 and the external drive used to store the pictures was connected to the Mac. Because the drive name, picture directories didn't change, I wrote a script to use the existing pathname field to insert the found picture into the FM container field that had no options set.
So, if I understand your answer correctly when I select "Store container data external", FM will transfer contents to the selected location. Won't this duplicate the existing pictures and directories?
When changing storage to external, pictures will be duplicated from source path to external container path. I hadn't tried 2 paths as same folder.