I'm not sure if I still understand what you are trying to do, but offer the following comments.
I would strongly suggest NOT to use the option to LINK to images outside of the FTM Media folder. That introduces the possibility that as the years go by, those files will be moved or deleted and you may not realize the link to those images inside of FTM is broken for years after. Besides, adding those images to the FTM image folder creates an additional level of backup.
My understanding, altough I admittedly haven't tested it, is that the FTM Media folder must be in the same directory as the FTM file it is for. For example, my main FTM file is at"
C:\Users\User\My Documents\Genealogy\Family Tree Maker Data\Tom2012\Tom2012.ftm
And associated Media folder is at:
C:\Users\User\My Documents\Genealogy\Family Tree Maker Data\Tom2012\Tom2012 Media\
The FTM file is usually so miniscule to the Media file that both should fit wherever you want them with no problem - ie your Dropbox folder. For example, my FTM file has 65,000 poeple and is only 200 MEGS, but my media folder is 12 GIGS, primarily census records. My FTM file is only a fraction of my FTM media folder.
The File > Backup option creates an archival backup with a date in it. I have changed my structures so that all of my archival backups are on removable media, primarily because they take up so much room on my C drive. In fact, I don't use the File > Backup option, but normally just copy the files themselves, with a date in the file name to the removable media. There used to be an advantage with compresssion via the File\Backup option, but that option does not compress my files hardly at all, so have basically stopped using it.
As a matter of note, I also backup my files with their normal names (a continuously written over backup - not an archival backup), with Carbonite and also an automatic backup at 1:00 each morning to an external hard disk.
In short, if you want your files in Dropbox, I would suggest put the whole kid and caboodle in the Dropbox folder (the folder can be designated under Options, as you pointed out.)