Check out all the CC licensing options here! NOTE: All content on H5P.org is licensed as Attribution CC BY (the first option) by default! Be sure to fill out copyright info for any images or media you are using!
You may have heard of Creative Commons or Pixabay, but there are LOTS of sources for public domain images! Be sure to doublecheck the copyright info on each site and for each individual image, if applicable. (Public domain means you are free to use the image without attribution or permission.)
This fee-based site was recently launched by H5P based on users request for a fully hosted solution with LMS support and robust tracking options. May be of interest to school or academic libraries who want to use H5P for more formal instruction.