I recently lost many watch faces when I backed up my BeautifulWatches folder using Android File Transfer (for Mac).
(I needed to do a factory reset of my phone).
Unfortunately, Android File Transfer skips files that start with . and thus I lost all the images for all my faces
when I restored (I was not aware that the files did not get transferred.)
In general, files starting with . are "hidden" so other tools will likely miss them as well.
I suggest using normal file names for the images so they can be backed up more easily.