The official RESTful Instagram API does not allow photos to be uploaded because they want to promote "life on the go."
However, some work has been put together to document their iPhone APIs, and some unofficial client libraries have already been done. Check out https://github.com/mislav/instagram/wiki
It seems your only alternative at the moment. None of the unofficial libraries look very complete, so you'll have to do the work of sending requests manually using the reverse-spec API specifications provided on the wiki.