Make the image field last to encourage proper workflow.
Add a clarifying note on image naming conventions.
Enforce ContentImage naming conventions, even via the Admin.

Previously, images were named according to this naming convention and
uploaded manually, and the importer parsed data from the image name
itself.  While the importer will no longer be used, this maintains the
image naming convention for the sake of clarity if admin users need to
manually examine images. (#500)