Imgembed is a “crowdsourced” collection of what’s now up to one million images that are free to use as long as you credit the photographer (it automatically appears with the embed code). You can also pay if you don’t want to have a header with an attribution. And you can contribute your own photos, too.

You can read more about it here at 10,000 Words, and I’ve embedded a short video about it below. I’m adding it to The Best Online Sources For Images.