There is a risk of liability for copyright infringement if you use a copyright work without permission. Publishers may refuse to accept a manuscript containing any third party material without clear authority.
The best option is to find an alternative, but if an alternative is not possible, include a “good faith notice” . A “good faith notice” should say how you can be contacted and indicate your willingness to negotiate payment and/or take down the material if the copyright owner comes forward.
For teaching materials, it may be possible to rely on Section 200AB to use an orphan work if there is no other alternative. Read more about copyright exceptions in the Copyright Support Brightspace site.