The files in the DropBox folder are NOT files, they are aliases. When you access them they are downloaded from the 'Cloud' or the DropBox server somewhere in outer space.
Mostly... I think on my computer if I place the original Filemaker file in the folder, it stays there but a copy is uploaded to the cloud for sharing.
You can see this when save an fmp file to the DropBox folder on your computer and when you access this file on IOS it must be DOWNLOADED and then saved and opened in the Filemaker folder.
Why not create a table for these images in a master file on your office desktop server, Pro or Server. Then your remote clients only need to sync with that file rather than with DropBox and you can automate this and eliminate operator error (but not developer...).
This is relatively easy to do.