In high school, I took a mini-workshop about making zines, and specifically finding images that have a Creative Commons license (non-copyright type things). I learned about this website, the NOAA Photo Library, which is part of the National Oceanic and Atmospheric Administration. They have an enormous photo library, full of high-quality photos that are free to use and download. There's also usually a lot of information about each photograph. So much fun!
The Inner Harbor, 1985
There is also the Creative Commons website which has it's own kind-of-search engine, that accesses multiple search engines for CC licensed content, which is really nice.