The file can be inserted from the iPhone without doing anything on the PC. Filemaker Go is free. You would have two options on the iphone using the insert from device.
1) The iphone uses open-remote to connect to the hosting database and insert the photo to the container. The method will require that the user is connected to the local network or has an internet connection to connect to the database.
2) Standalone database that the photo is inserted into on the phone and then when the user is connected to the internet or to the local network then you export the data / sync the data to the main database.
See : Insert From Device http://www.filemaker.com/help/13/fmp/en/html/scripts_ref1.36.47.html
Filemaker Sync Guide http://www.filemaker.com/solutions/ios/docs/fmi_guide_sync_en.pd
Seedcode has a sync program for filemaker. The free version will only sync one table, which may be all you need. You would just sync the photo table.
As an alternative, you might try using Open URL with a file path that specifies the path to the enclosing folder on their PC. If they choose the options that sort the file contents in descending order by creation date and display a thumbnail for each image file, you might be able to set up a system where they use drag and drop to add the photo to the database file on the PC.