A common requirement for blogging applications (and one I haven't implemented) is the ability to upload images to a blog.
The MetaWeblogAPI does provide a mechanism to do this: the newMediaObject method.
Unfortunately this was until recently notable for the fact that precious few blogging engines implemented it (which is why I haven't done anything about it).
...But now, well, things are changing: recent releases of both Community Server and dasBlog for example do reportedly implement it. The fly in the ointment is the inertia of installed software: how to know if any given installation you're interacting with implements it or not.
It's not reasonable to expect the user to know, and do we want them to find out by trial and error?
It's a tough one, and if anyone has any good ideas on the subject please feel free to contact me and let me know.