The Creative Commons suite includes licenses that implement various policies. Some, like CC BY and CC BY-SA, are normatively consistent with corresponding permissive and copyleft families of free software licenses. Others, however, particularly its "NC" (no commercial use) and "ND" (no derivative works) licenses, are in conflict with basic principles of free software and free culture. I am not alone in lamenting the application of the Creative Commons umbrella brand to cover licenses with such disparate qualities. One consequence has been a general confusing dilution of the meaning of "openness" in the context of cultural works. A more specific problem is the evidence of confusion on the part of content authors interested in applying Creative Commons licenses to their works, and resulting confusion by those interested in making use of such works. Too often a work is labeled as being licensed under "a Creative Commons license", without specifying accurately, or specifying at all, which free or nonfree policy the author sought to apply.
If you still cannot find content--images, for example--with free licenses, but you find copyrighted content that fits your academic need, you can reach out to the content creator or copyright holder for permission. Often copyright holders allow usage of their work for non-commercial purposes, such as academic research and publication.
Do you have other sources you recommend for finding Creative Commons or Public Domain content? Let us know about your favorite resources in the comments.