There are always exceptions, so I'm sharing guidelines and principles rather than "rules," but years of promoting my stories online and closely following the performance of stories other P&P writers produce do tend to show me clear trends. I don't know that Ansel Adams ever tried competing for attention in a social media feed where he had only fractions of a second to capture interest and produce a click.

When I try that with a black and white image, the results are usually (but not invariably) disappointing.