I'm pretty averse to using the downvote, so for me this would depend on context. If it's literally just a screenshot with "how?" written after it, yeah, downvote. But in general I think downvoting has more power to harm a community than to help, so I use it with caution.
If there's at least a little attempt to explain and articulate then I wouldn't vote one way or the other, but would either in a comment or answer, indicate that screenshots are strongly discourage (if for no other reasons, the site depends on keyword searches to help cross-reference questions and answers, so that others can benefit from the answers). Of course if a person continues violating the rule, then downvote and flag.