When preparing presentations I occasionally need a specific photo to underline the point I am trying to make. As I am not a photographer by any stretch of the imagination, I usually use the work of people who are better at taking pictures than I am.
That presents a challenge in itself. I would like to use good quality stock photography, but I am unwilling to break any copyright law to do so. For those instances where there is no budget to purchase stock pictures the folks over at Slideshare point to a number of resources where you can download photographs free of charge.
All photos from the following resources have been made available under a Creative Commons CC0 license. That means you can copy, modify, and distribute them without asking permission, even for commercial purposes. However, including a reference to the photographer would be the courteous thing to do.