I had this same problem when converting my file in 2008. I lost 100's of images due to unknown file types (that were actually jpeg2000 files) that version 2008 cound not translate properly. Fortunately, later, FTM fixed the issue in 2011 but it was too late for my file.
You might try adding a suffix (like .bmp, .png, .img, .tif, etc...) to a file and see if the file can be read. Or, you might have to go back to your version 16 file and copy/paste the images to the new FTM Media folder and relink them in 2012.
Some of the files have names associated with them so finding the file in the v16 file would not be difficult but will take some time. Version 16 did not handle images in a good way. For example, look at the image named "Tyler and Abigail Webb 2012." You can read this image in version 16. Opening the file in version 16 brings up "Paint." The file can be saved to a new location and relinked.
Edit: FTM 2012 has a report called a "Media Report." You might be able to run that and see the names of the persons attached to each of the files.