Some can be tagged correctly without the original uploader:
- fair use
- where a suitable source is given
- images that are obviously PD because of age
and probably some other cases.
That said, the {{unverified}} template should probably include a warning not just to guess a license, and that doing so might have legal consequences. After all, if I tag a copyrighted picture with, say, {{GFDL}}, and wikipedia (or someone else using it) gets sued, they can point to me for tagging it, right?
That sounds perfect to me.