Photo procurement

I'm writing a non-fiction book. I see many photos on the Internet that I would like to use in my manuscript. I need charts, maps and diagrams as well. Sometimes these photos have credit headings of the author or photographer, but there's really no way to contact them.
Who is the best source for asking permission to reprint?
The web master? Or would it be the society or association itself who displays this material?
I'm looking for stock drawings or art renderings of prehistoric ice age animals, along with geologic time tables and related charts.

Triceratops

It would most likely be the society or assocation which owns the site. Somewhere on the site should be a copyright notice or a statement like "all rights reserved" followed by the name of whoever/whatever is claiming same. I'd direct queries to the webmaster only if no other contact info is available and then only to ask for the contact info of the proper individual/organization.