Does anyone have any good photos, descriptions or know any good literary resources for nordic depictions or descriptions of the supernatural? I'm thinking something like the wraiths seen sometimes in guldgubbers, or other weird anthropomorphic figures (Not the wolf or bird-headed warriors, mind). Any ideas?

Grettir's saga has ghosts, trolls, and giants: http://www.gutenberg.org/files/347/347-h/347-h.htm#link2HCH0032. It doesn't spend a lot of time describing their appearance.

I would suggest looking under Valkyrie. http://en.wikipedia.org/wiki/Valkyrie

There are some interesting paintings from much later period, and some archeological depictions to start from.

There are somewhat more detailed descriptions of a draugr in Eyrbyggja Saga, but what is there is still fairly sketchy. There is plenty of the supernatural in Volsunga Saga as well.