This is my latest mashup video. I've just begun playing around with Flickr photos that are licensed under Creative Commons licenses. There are many millions of photos shot by people--probably many of whom are amateurs--that you can reuse for noncommercial purposes with attribution. There are also millions of photos that you can reuse for even commercial purposes as long as attribution is given to the photographer. You can explore the Creative Commons photos by clicking on this link. (The searches are broken down into the type of CC license granted for the photo.)
The Creative Commons community on Flickr is pretty amazing. The photos in this video are just a sample of the millions of truly impressive photos on Flickr.