Images which have been made Creative Commons Zero, also known as CCO, are available to use by anyone, however they like. The images are in the Public Domain and can be reproduced, incorporated into other works, modified, and reused, without needing permission and in some cases without even needing to credit the author.
A number of image sites offer CCO works. They are in two broad categories - artworks, and stock photography. The latter may be useful for pepping up presentations or websites; examples of the former are listed below and may be directly useful as part of your degree.