For the time-being you can only do this as you describe (one cover at a time).
The reason for this is that the covers are chosen by a completely automated process and therefore it is possible that it might be wrong; I didn't want to run the risk of automatically inserting incorrect covers into people's files, and so opted to implement the manual transfer only.
I do have ideas, however, for making a better interface for manual transfer, which should make it a lot quicker to do large batches of transfers in a small amount of time.
You are posting anonymously so your submission will be held for moderation.
to avoid this delay.