By free content I mean CC0 (Public Domain).
You can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission. See Other Information below.
Here is a list of CC0-licensed awesome stock photography.
My favorites is unsplash.com.
If you are ok with giving credit you can expand your search for content by including Creative Commons. Mostly known by the license CC BY 4.0.
Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.