I'm afraid that we can't really provide legal advice. There may be provisions such as "fair use" that allow for use of copyright-protected work, though these will vary from country to country; it may or may not be enough that the work is not used for profit and is used in an educational context. I would suggest that the safest course of action is to have your students produce their own artwork or use images from sources that expressly allow re-use. You can use CC search for example to find such images.