index.xml is fine for what it is, but (by the necessity of the problem it
solves) it lists many of the files multiple times, and of course it does not go
in package/directory order.
The manifest.xml should do the following:
* List each and every file in the package exactly once, including directories.
* List associated files together, e.g., list each .svg file's associated
thumbnail along with it, probably as an attribute.
* List them hierarchically, i.e., the files in any given directory together,
nested within the element corresponding to the directory.  The root element
should correspond with the root directory and so directly contain elements for
special files such as LICENSE.txt and the manifest itself.
* List *some* metadata about the file, including size (in bytes).
* For the clip art images themselves, list additional metadata, such as title
and author; these probably should be listed as attributes.  Not sure whether
keywords should also be listed, as those would probably require elements, can be
numerous, and so could lead to a larger manifest than is really necessary.