Curator will not replace existing custom icons. If a folder has the wrong artwork, you can choose Artwork  ▶ Delete Custom Icon to remove the incorrect one, and then Artwork  ▶ Create Custom Icon to make a new one.

Curator 1.1.4 improves the resolution of icons from 128 to 512 pixels. On Mac OS X Leopard, icons created by previous versions of the program may look grainy. You can hold down the option key when choosing the above commands to delete and create icons for all folders.