Photos presented in the Haiku Deck image search are all licensed under the Creative Commons license. This license has a variety of flavors. This license gives everyone from individual creators to large companies and institutions a simple, standardized way to grant copyright permissions to their creative work. The combination of the Creative Commons tools and its users amounts to a growing pool of content that can be copied, distributed, edited, remixed, and built upon, all within the boundaries of copyright law.
You can see the image license by clicking on the cc logo in the top left corner of slides when you're in playback mode. In the illustration below, the photographer attribution link will connect you to the source of the photo. This includes detailed information about the photo and associated license. Other icons shown provide a shorthand for understanding that this particular photo is licensed under the Creative Commons for non-commercial use (that's what the $ with a slash through it).
Here's the same slide shown as a .pptx export. Note the image license and attribution is shown as a footnote along the bottom of the slide.