Export Field Contents can export these images as individual files. A script could thus export all such images by looping through all the records in your table.
The Problem is when i Right-click on the image in the container field, it has a greyed-out "Export Field Contents". Edit Menu also display a grey-out "Export field Contents".
I just ran a quick test and confirmed that items inserted as an object (what happens when you drag and drop a file from outside the Filemaker window), cannot be exported with export field contents.