You can find many open resources on the mega sites listed below. Please note that varying degrees of "open" exist between, and even within these sites. Remember to assess each individual resource for its appropriateness, accessibility, and level of openness.
Search these resources to find openly licenced or public domain images.
There is a growing number of free courses available online for students to explore. As well, open educational resources can provide students with supplemental materials for their courses.
"Open Education Resources (OERs)" by TRU Libraries is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).
Based on a work at http://open.bccampus.ca/bcoer-librarians/bcoer-guides/.