Thank You, now You can safely delete items from dropbox.
Looks like these files do not have artwork embedded. Desktop Vox
probably manages to find these on the hard drive or internet for
You, iOS version, for some reason we are investigating,
doesn't.
What You could do is Google artwork picture (.jpeg or .png),
does not need to be large, 512 x 512 px maximum, download it and
place it in the same folder as Your album. Assuming there is one
album in one folder, You will be able to add cover for every album
that is missing it.
Vox will try to pick up pic in folder as album's artwork. Of
course You will have to re-upload albums to Vox via iTunes File
Sharing.