Is downvoting a post that is a photo I can trace to google, or a post that is just a youtube video helpful?

It really depends, first of all, your downvote will only be as strong as your voting power allows, so flagging a user who is more reputable and "stronger" than yourself will have only a minor effect. But still, you may use your voting power freely to down- or upvote content based on your own judgement. Sending a message to spammers and copy-cats certainly isn't the wrong thing to do.

am I doing any harm continuing to vote on quality posts once my voting power is drained?

No, no harm done, but without voting power, you are also not contributing much either.