If it carries information, it's valuable. Sometimes pictures really don't illustrate what they're supposed to be illustrating, and sometimes they do. That's linking by association . By authority I mean author -ity. A printed document many times has gone through peer review, probably has at least gone through an editing process and is in some ways substantiated.
Most publishers check information before they put it out.
With Web documents, you don't even know if they're legitimate. They could be multiply authored. They could be fake. They could be horrendous information. There's all kinds of wonderful and horrible things out there.