But I think we have to ask ourselves, every time we use content that has not been freely licensed, is this something we're ready to *pay* to go court over?
This is a different question from asking whether we'd win any given case. I assume without much uncertainty that we'd win a fair number of them -- in theory. But we're not in the position of going to court to win even the cherrypicked, obviously winnable cases.
When assessing risk, you need to consider not only the cost of something going wrong (in this case, clearly quite high), but also the chance of it happening. How far would a policy of complying with all but the most outrageous cease and desist notices go towards reducing that chance? In other words, would be a good idea to not worry about the borderline fair use cases until someone complains? What's the chance of us ending up in court even after removing the image in question?