Please read for an updated status on RailsCasts:
Learn more or hide this
GitHub User: milgner
Just a quick update: the two problems with the save event not getting fired and image drag&drop not working will be resolved if you update to the latest version from the git repository. (http://jejacks0n.github.com/mercury/) Note that you will have to rename your table to mercury_images instead of images if it already exists.
Same issue here. Did you find a solution to this problem?