Several images I've encountered have an impossible obstacle to overcome with regards to provenance. Specifically, their purported subjects' lifespans predate the emergence of portrait photography. When asked about the provenance of the image, contributors, rarely answering at all, usually simply provides their modern source of dubious reliability. The purpose of the question is to ascertain if the photograph (or other medium) reliably depicts the person on the tree. A couple of articles below may be helpful if terms or history are unclear:
http://en.wikipedia.org/wiki/Provenance
http://en.wikipedia.org/wiki/History_of_photography
I hope to add tags here if provenances are challenged yet remain unestablished.
Let me also "advertise" the Project you started, Adam.
http://www.geni.com/projects/Provenance-and-Images
Anyone who would like to join it can request to do so. I am all for cleaning up images in the tree.